Different composition poly(methyl methacrylate-co-butyl methacrylate) copolymers through seeded semi-batch emulsion polymerization

Abstract

In the present work the synthesis and the chemical and thermal characterization of poly(methyl methacrylate-co-butyl methacrylate)
copolymer, in three different macromolecular compositions, are reported. The aim of the present work was the identification
of a standard method to obtain copolymers with controlled macromolecular composition, molecular weights and particle size
distribution, together with the identification of the effect of the macromolecular composition on the material properties.
A monomer-starved seeded semi-batch emulsion reaction was carried out and optimized, monitoring the kinetic of the copolymerization
through the evaluation of residual monomer amounts. Then, an evaluation of the macromolecular composition was performed by
Fourier transform infrared spectroscopy analysis. Molecular weight, molecular weight distribution, latex characteristics and
thermal behaviour were also investigated.

Full text

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ORIGINAL PAPER
Different composition poly(methyl methacrylate-co-
butyl methacrylate) copolymers through seeded
semi-batch emulsion polymerization
Davide Silvestri Æ Æ Mariacristina Gagliardi Æ Æ
Caterina Cristallini Æ Æ Niccoletta Barbani Æ Æ
Paolo Giusti
Received: 21 January 2009/Revised: 22 April 2009/Accepted: 27 April 2009/
Published online: 5 May 2009
? Springer-Verlag 2009
Abstract
acterization of poly(methyl methacrylate-co-butyl methacrylate) copolymer, in
three different macromolecular compositions, are reported. The aim of the present
work was the identification of a standard method to obtain copolymers with
controlled macromolecular composition, molecular weights and particle size dis-
tribution, together with the identification of the effect of the macromolecular
composition on the material properties. A monomer-starved seeded semi-batch
emulsion reaction was carried out and optimized, monitoring the kinetic of the
copolymerization through the evaluation of residual monomer amounts. Then, an
evaluation of the macromolecular composition was performed by Fourier transform
infrared spectroscopy analysis. Molecular weight, molecular weight distribution,
latex characteristics and thermal behaviour were also investigated.
In the present work the synthesis and the chemical and thermal char-
Keywords
Radical polymerization ? Poly(methyl methacrylate-co-butyl methacrylate)
Monomer-seeded semi-batch emulsion polymerization ?
D. Silvestri ? M. Gagliardi (&) ? N. Barbani ? P. Giusti
Department of Chemical Engineering, Industrial Chemistry and Materials Science,
University of Pisa, Via Diotisalvi 2, 56126 Pisa, Italy
e-mail: mc.gagliardi@ing.unipi.it
D. Silvestri ? P. Giusti
Interdepartmental Centre for the Study and Evaluation of Biomaterials and Endo-Prosthesis,
‘‘Nicolino Marchetti’’ (C.I.B.E.), 56100 Pisa, Italy
C. Cristallini
CNR Institute for Composite and Biomedical Materials IMCB,
Pisa c/o Department of Chemical Engineering, Industrial Chemistry and Materials Science,
University of Pisa, Via Diotisalvi 2, 56126 Pisa, Italy
123
Polym. Bull. (2009) 63:423–439
DOI 10.1007/s00289-009-0095-2

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Introduction
Copolymers are widely employed in industrial processes because their physico-
chemical properties, like strength, elasticity or thermal behaviour, can be modified
by varying their macromolecular composition. The final functional versatility of
these materials takes also origin from the possibility to carry out the synthesis by
chemical reactions involving different monomers with different reactivity ratios and
properties.
Industrial copolymerizations are based on systems implicating the presence of
two or more monomers generally with high water solubility [1, 2] as carboxylic
monomers, used in the production of particles with reactive groups on their surface
[3, 4]. These monomers have a specific role in imparting a colloidal stability of
dispersions used in the production of adhesives and binding agents and films and
membranes with specific mechanical properties. In this contest, emulsion polymer-
ization is a widely used technique employed in industrial production of latexes,
paints, coatings, adhesives and bindings [5]. Acrylic polymers are a group of
materials obtained by emulsion polymerization, where n-butyl acrylate (n-BA),
n-butyl methacrylate (n-BMA) and methyl methacrylate (MMA) are the most
commonly used monomers.
Final systems are widely versatile and many latexes, showing several
applications as coatings and adhesives, can be produced by varying the molar
ratio between comonomers. Final properties of these latexes highly depend on the
micro-structural properties of the polymer and, for this reason, it is essential to
understandthe physicochemicalphenomena
copolymerizations.
Previous studies [6–8], focused on the n-BA/MMA semi-batch emulsion
copolymerization, evaluated the effects of various reaction parameters on the
particle nucleation and growth, also through mathematical models to describe the
particle growth and the mechanism of particle formation. In this work, in order to
control the effective final macromolecular composition of synthesized materials,
copolymerization was carried out via a semi-batch emulsion polymerization under
monomer-starved conditions [9–13] that allows controlling also the temperature of
the reactor [14] and the particle size distribution [15].
From literature [16–20], copolymers composed of methyl methacrylate and
n-butyl methacrylate are very interesting. Monomers chemical structures resemble
each other and a similar behaviour can be expected. In practice, homopolymers
obtained from these monomers are very different in chemical, mechanical and
thermal properties.
In the present work an optimized method suitable to control the composition
during the synthesis, the effects of the comonomers ratio on the monomer
consumption and effects due to the macromolecular composition on the thermal
properties were studied. The copolymer poly(methyl methacrylate-co-butyl meth-
acrylate) in three different molar compositions was synthesized and characterized in
order to evaluate the instantaneous composition of the reactive mass, the molecular
weight, the molecular weight distribution, the latex properties and the thermal
behaviour.
involvedin the emulsion
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Experimental section
In the present work, three different composition materials were synthesized by
varying the feed used for the reaction. Starting percentage molecular feed
compositions of MMA/BMA used were: 87.5/12.5, 75/25 and 50/50, polymer
nomenclature was referred to the feed reactor molar composition. Also, homopoly-
mers PMMA and PBMA were synthesized with the same scheme used for
copolymer synthesis with the aim of comparing copolymers properties with
reference homopolymers and evaluating effects of composition on the properties of
newly synthesized materials.
Copolymer synthesis
Materials
Materials were synthesized in a seeded semi-batch emulsion copolymerization
under a monomer-starved condition and the same scheme was used for the synthesis
of homopolymers.
Methyl methacrylate and n-butyl methacrylate, both obtained from Sigma
Aldrich, were purified to remove the hydroquinone inhibitor before the use. The
monomers were afterwards stored in the dark at -20 ?C and used within 2 weeks.
Potassium persulphate (KPS, Lancaster?, purity grade: 99.99%) was used as
initiator and sodium dodecyl sulphate (SDS, Sigma Aldrich?) as emulsifier agent.
Bidistilled and deionised water was used as emulsion medium for all reactions. KPS
amount introduced within the reactor was 0.027% (mol/mol) with respect of the
total monomer moles amount, SDS introduced was 0.25% (w/v) with respect of the
total liquid amount in the reactor, constituted of water and monomers. The reactions
were carried out with constant number of monomer moles (0.25 mol) and total
liquid volume (500 ml), then SDS amount was 1.25 g and KPS amount was 0.018 g
for all reactions. Amounts of MMA, BMA and water employed for reactions were
reported in Table 1.
Polymerization apparatus
Reactions were carried out in a three-neck round-bottom flask immersed in a
thermostatic oil-bath. An Allihn reflux condenser, refrigerated with water at room
temperature, was used to avoid the loss of monomers. A paddle equipped with a
Table 1 Recipes of the
monomer-seeded semibatch
emulsion copolymerizations
MaterialMMA
(mol)
BMA
(mol)
H2O
(ml)
PMMA 0.25– 473.27
P(MMA-co-BMA) 87.5/12.50.219 0.031471.63
P(MMA-co-BMA) 75/250.1880.062 470
P(MMA-co-BMA) 50/500.125 0.125466.75
PBMA– 0.25 460.24
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Teflon?impeller was used, connected to a Heidolf RZR 2020 engine set to
260 rpm. Reaction was carried out under continuous N2flux. Temperature was
uniformly maintained at 75 ?C and controlled by a thermocouple trough all reaction
period. The reaction was divided in three parts: during the first, both monomers
MMA and BMA were fed, in the second only MMA monomer was fed and in the
third the reaction was allowed to proceed without any feed. Each part of the reaction
lasted 1 h and then total reaction time was 3 h.
Polymerization method
The SDS emulsifier was dissolved in 400 ml of water and, after the complete
dissolution, was introduced in the reactor and heated at 70 ?C. The KPS initiator
was dissolved in the rest of the water with respect of the Table 1. A seed of 32% (w/
w) of monomers was added to the initial reactor charge and purged with nitrogen for
10 min then the temperature was increased to 75 ?C. When the temperature reached
75 ?C, KPS solution was introduced in the reaction mixture. From this point, the
feed was added in spot every 15 min for eight times (see Table 2). Four additions of
both MMA and BMA monomers were carried out in the first hour, while four
additions of only MMA monomer were carried out in the second hour of reaction.
This parcelling was selected in order to avoid having a great amount of MMA in the
reactor during the first period of reaction of copolymers 87.5/12.5 and 75/25. For
analogy, the same scheme was adopted for copolymer 50/50.
Polymerization was stopped by rapidly reducing the temperature at 25 ?C using
an ice bath.
Study of the monomer consumption
During the polymerization, samples were withdrawn for off-line analyses from the
polymerization mass at regular intervals of time. Samples were withdrawn
immediately after the feed and then after 1, 2, 5 and 10 min. 5 ll were withdrawn
and diluted in 5 ml of bidistilled and deionised water. Reaction was sharply blocked
by decreasing the temperature using ice and then putting the sample at -20 ?C.
Samples were analysed by High Performance Liquid Chromatography (HPLC) to
evaluate the effective amount of residual unreacted monomers present in the reactor.
Table 2 MMA feeds were
introduced within the reactor for
eight times every 15 min after
the initial charge during the first
and the second part of the
reaction (2 h), BMA feeds were
introduced for four times every
15 min after the initial charge
MaterialInitial
charge
(mol)
MMA
step-by-step
feeds (mol)
BMA
step-by-step
feeds (mol)
PMMA0.080/– 0.021–
P(MMA-co-BMA)
87.5/12.5
0.070/0.0100.019 0.005
P(MMA-co-BMA)
75/25
0.060/0.020 0.0160.011
P(MMA-co-BMA)
50/50
0.040/0.0400.011 0.021
PBMA –/0.170–0.043
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Withdrawn samples contained MMA and BMA monomers, SDS, KPS and water,
therefore a chromatographic method was necessary to separate all components. A
chromatographic method was developed expressly for this aim, inspiring to the
work proposed by Sofou et al. [21]. HPLC apparatus consisted in a 410 LC pump,
an ALLTECHC18 3Ucolumnwith
100 mm 9 4.6 mm) and a UV detector (Perkin-Elmer). Reagents used were
specific HPLC grade acetonitrile (ACN, Carlo Erba Reagenti?) and double
deionised water. The mobile phase was constituted by 80% ACN and 20% water,
test time was 10 min, internal flux rate was 0.8 ml/min, injected volume was 50 ll
and wavelength was 210 nm at room temperature.
a non-polarphase (dimensions
Characterization
After synthesis, materials were analyzed in order to evaluate the chemical and
morphological characteristics, the molecular weight, the effective macromolecular
composition and the thermal behaviour.
The polymeric powder was isolated from the latex structure drying the latex in a
vented oven at 40 ?C for 24 h. The dried powder was washed with hot water (70 ?C)
to eliminate the SDS; the solid content was recovered from the water and then dried
again. Copolymers were also washed with methylene chloride to eliminate the
PMMA homopolymer and then with isopropanol to eliminate the PBMA
homopolymer eventually produced in the copolymerization reactions. After these
purifications, the solid content was recovered and dried in a vented oven at 40 ?C
for 24 h before the use.
Morphological analysis and latex characterization
A morphological analysis was performed onto the dried latex and the washed
polymeric powders to evaluate the particle size and the particle size distribution.
Analyses were carried out through Scanning Electron Microscopy (SEM) JEOL
JSM 5600.
Also, a characterization was performed on the latex. In particular, the
experimental and the theoretical solid contents were evaluated. Theoretical
(Eq. 1) and experimental (Eq. 2) solid contents of the latex were evaluated as [22]:
SCth% ¼weight of introduced solid (g)
sample weight (g)
? 100
ð1Þ
SCexp% ¼weight of dried latex (g)
sample weight (g)
? 100
ð2Þ
These values were calculated taking the presence of the SDS amount into
account.
Molecular weight and molecular weight distribution
Mean molecular weights (number, viscosity, weight and z) and dispersion indexes
were determined by gel permeation chromatography (GPC) using a Perkin Elmer
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apparatus, equipped with a LC 90 UV spectrophotometric detector, a Series 200
refractive index and one ResiPore column (dimensions 300 9 7.5 mm, particle size
3 lm). Tetrahydrofuran (THF, Lab Scan?, HPLC purity degree) was used as solvent
at a flow rate of 1 ml/min. Standard calibration curve was obtained analyzing
narrow polystyrene standards with known molecular weights using appropriate
Mark-Houwink-Sakurada constants [23] and TurboSEC software. For synthesized
homopolymers, the Mark-Houwink-Sakurada constants were obtained from liter-
ature [24] and, supposing that copolymer macromolecules were linear, a linear
interpolation of literature data was performed in order to obtain k and a values of
copolymers.
Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy (FT-IR) analyses were carried out by a
Perkin Elmer Mod Spectrum GX. FT-IR analysis allowed evaluating the effective
macromolecular composition of copolymers, identifying characteristic bands for
homopolymers within the copolymers spectra.
Chemical analysis
To chemically characterize synthesized copolymers,1H NMR analysis was carried
out.
1H NMR spectra of PMMA, PBMA and P(MMA-co-BMA) copolymers were
measured at room temperature and 200 MHz, with NMR Varian-Gemini 200
Spectrophotometer, starting from 2% solutions of copolymer in deuterated
chloroform, using 5 mm diameter tubes and the solvent as standard.
Thermal analysis
The glass transition is considered one of the most relevant characteristics to
appreciate the practical use of amorphous polymers [25]. Glass transition
temperatures (Tg) for synthesized copolymers were evaluated by Differential
Scanning Calorimetry (DSC). Calorimetric analyses were obtained using a Perkin
Elmer DSC 7 instrument. Samples (4 ± 0.2 mg of polymeric powder) were sealed
in aluminium pans. The thermal behaviour of the materials was followed from 0 to
200 ?C (heating and cooling) at a rate of 10 ?C /min. The results of the second
heating scan were considered. Nitrogen, with a flow rate of 10 mL/min, was used as
a purge gas during the scans.
Results and discussion
Synthesis and monomer consumption
In order to describe the progress of the reactions, the standard assumption of well-
mixed reactor was considered. When the reaction started, the reactive mass within
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the reactor was completely transparent, during the reaction it came firstly bluish and
then white. The presence of the surfactant and the high-stirred conditions ensured
the formation of very small polymeric particles that gave rise to the formation of a
stable latex.
For a semi-batch reaction, monomer conversion is defined as the number of moles
ofmonomerreactedintothereactordividedbythetotalnumberofmolesofmonomer
loaded and fed to the reactor [14, 15, 26–28]. According to this definition, monomer
conversions were evaluated and in Table 3 the final conversions is reported.
Also, the overall (Eq. 3) and the instantaneous (Eq. 4) conversions, indicated
respectively as xovand xin, were evaluated as [27]:
xov¼moles fed at time t ? unreactedmoles
total added moles
xin¼moles fed at time t ? unreactedmoles
moles fed at time t
In Fig. 1, xovand xinfor the P(MMA-co-BMA) 75/25 copolymer, are reported.
From Fig. 1 it is possible to observe that the most important condition in a semi-
batch reaction, that is the reaching of a high conversion before the addition of fresh
monomers, was respected.
Monomer-starved conditions proposed for the synthesis of these materials imply
that the monomers are added in the reacting mass with a rate similar to the
polymerization rate [29]. For this reason, is important to have a fast reaction and the
time elapsed between feeds has to be enough to permit the almost complete
consumption of the monomers. Monitoring the monomer disappearance is possible
to establish these parameters. A trial-and-error procedure was executed before to
reach the scheme proposed in this paper that was able to inflect the macromolecular
ð3Þ
ð4Þ
Table 3 Conversion values of monomers
Material Fed molesResidual moles Final conversion
PMMA
MMA 0.25 1.27 9 10-4
0.990
BMA–––
P(MMA-co-BMA) 87.5/12.5
MMA 0.2191.17 9 10-4
2.07 9 10-4
0.999
BMA 0.031
0.993
P(MMA-co-BMA) 75/25
MMA0.1881.72 9 10-4
6.63 9 10-5
0.999
BMA 0.063
0.999
P(MMA-co-BMA) 50/50
MMA0.1254.82 9 10-4
8.02 9 10-5
0.996
BMA0.125
0.999
PBMA
MMA––
1.93 9 10-5
–
BMA0.25
0.999
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composition. The amount of monomers present in the reactor was monitored off-line
by HPLC, using the chromatographic method previously reported. In order to save
space, only results relative to P(MMA-co-BMA) 50/50 copolymerization are
showed in this paper (Fig. 2). For all reactions results are similar.
The amount of monomer added in each feed polymerized almost immediately
after the introduction in the reactor. This remark was of paramount importance to
ensure the monomer-starved condition and in consequence to control the
composition of the copolymers obtained in the semi-batch reaction.
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 1530 45 6075 90 105120 135150165 180
Time [min]
Overall conversion
MMA
BMA
0,000
0,100
0,200
0,300
0,400
0,500
0,600
0,700
0,800
0,900
1,000
0 15 30 45 6075 90 105 120135150 165 180
Time [min]
Instantaneous conversion
MMA
BMA
Fig. 1 Overall (top) and instantaneous (bottom) monomer conversions evaluated by HPLC analysis. In
this figure results obtained for the copolymer P(MMA-co-BMA) 75/25 are reported
430 Polym. Bull. (2009) 63:423–439
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In effect, to take full advantages of semi-batch emulsion polymerization, it is
necessary to have a monomer-starved condition to prevent accumulation of
monomers in the reactor. For this reason, a continuous monitoring of the residual
monomers concentration was important to ensure the mentioned starved condition.
Characterization
Latex structure
Conventionally, emulsion polymerizations are used when monomers are hardly
soluble in water. However, this procedure is also used in many industrial
polymerizations involving the use of highly soluble monomers, as carboxylic
monomers. In this work to obtain an emulsion polymerization and a latex structure a
great amount of emulsifier agent SDS was introduced in the reactor (0.25% w/v with
respect of the total liquid amount). The presence of the surfactant and the high-
stirred condition within the reactor ensure the formation of a time-stable latex,
constituted of non-settle particles. Dimension of particles was evaluated by SEM. In
Fig. 3 a SEM image of particles obtained during the P(MMA-co-BMA) 50/50
copolymerization is reported as example. By the SEM analysis, a mean value of the
particle dimension was evaluated together with the polydispersity index, defined as:
Xnumber of particles with diameter i
Dimensions of particles are included between 200 and 500 nm for all synthesized
materials. Results are summarized in Table 4. It was not notice a trend of these
values in function of the feed compositions.
PDI ¼
total number of particles
? diameter i
ð5Þ
0
1
2
3
4
5
6
7
0 20406080 100120 140160 180
Time [min]
Residual monomer [g]
MMA
BMA
Fig. 2 Amount of residual monomers (g) in the reactor versus time for the copolymerization of P(MMA-
co-BMA) 50/50
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Latex can be characterized determining the solid content present in the reacted
mass after the stripping of the unreacted monomers.
Experimental values for the solid contents are very similar to theoretical. From
the latex characterization was possible to evaluate the gravimetrical conversion of
monomers. Gravimetrical conversion was evaluated on the total amount of
monomers in the formulations and then represents only a global conversion.
Gravimetrical conversion was evaluated from Eq. 6 and obtained results are
summarized in Table 5.
Gravimetrical conversion ¼SCexp%
SCth%
ð6Þ
Molecular weights
Emulsion polymerizations generally lead to obtain products with high molecular
weight and high dispersion index [22]. In the reaction system applied, the
composition of the monomer mixture is equal to the desired copolymer compo-
sition. In this work molecular weights were evaluated through chromatographic
analyses by GPC. Using the calibration curve, constructed as previously described,
polymeric solutions (5% w/v) in THF were prepared and analyzed.
Mark–Houwink–Sakurada (MHS) constants for polystyrene and for tested
materials are reported in Table 6.
A copolymer with a given composition and a maximum polymerization rate was
the main target of the present work and to maintain the desired copolymer
composition we needed to keep the corresponding composition of the reacting mass
under control by adding the monomers at an appropriate rate, maintaining at any
rate a final high molecular weight of polymeric materials.
Fig. 3 SEM picture of the particles obtained during the copolymerization of P(MMA-co-BMA) 50/50
432 Polym. Bull. (2009) 63:423–439
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Molecular weight resulted to be not affected by the macromolecular composition
because a trend with the macromolecular composition was not highlighted.
Molecular weights obtained are high and it can be explained by the great radical
concentration within the formed particles with respect of the rest of the mass,
constituted substantially of water. When the step-by-step monomeric feed was
introduced, radicals were active in the particles and a greater possibility of a chain
growth, with respect of the formation of a new chain, occurred.
Table 4 Mean diameter of the
particles obtained and
polydispersity index, evaluated
using Eq. 5
Material Mean diameter
(nm)
Polydispersity
index
PMMA315 0.76
P(MMA-co-BMA) 87.5/12.5321 0.83
P(MMA-co-BMA) 75/253290.74
P(MMA-co-BMA) 50/50312 0.82
PBMA346 0.79
Table 5 Theoretical and
experimental solid contents of
the latex and gravimetrical
conversions
MaterialSCth% SCexp% Gravimetrical
conversion
PMMA5.02 5.02 1.000
P(MMA-co-BMA) 87.5/12.55.295.28 0.999
P(MMA-co-BMA) 75/25 5.585.57 0.999
P(MMA-co-BMA) 50/506.09 6.091.000
PBMA 7.177.13 0.995
Table 6 Molecular weights: number (Mn), viscosity (Mv), weight (Mw) and z (Mz), dispersion indexes
(DI = Mn/Mw), Mark-Houwink-Sakurada (MHS) constants and intrinsic viscosities (g) evaluated by GPC
Material
Mn(Da)
Mv(Da)
Mw(Da)
Mz(Da) DI (Mn/Mw)
Molecular weights and DI
PMMA 433,000784,000840,0001,243,0891.94
P(MMA-co-BMA) 87.5/12.5383,000802,000 866,0001,296,1582.26
P(MMA-co-BMA) 75/25 430,000840,000 907,000 1,343,791 2.10
P(MMA-co-BMA) 50/50 518,0001,030,0001,110,000 1,564,1852.15
PBMA 394,000 1,050,0001,160,000 1,762,7062.95
Material
k (dl/g)
ag (dl/g)
MHS constants and intrinsic viscosity
PMMA9.44 9 10-5
1.05 9 10-4
1.12 9 10-4
1.24 9 10-4
1.48 9 10-4
0.719 1.633
P(MMA-co-BMA) 87.5/12.5
0.7111.657
P(MMA-co-BMA) 75/25
0.704 1.658
P(MMA-co-BMA) 50/50
0.691 1.770
PBMA
0.6641.476
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Fourier transform infrared analysis
FT-IR analysis allowed evaluating experimentally the actual macromolecular
composition from the spectra of the tested materials. Within the spectra of the
homopolymers, a characteristic band for PMMA was identified at 2,994 cm-1
(corresponding to the asymmetric stretching of the CH3bond) and for PBMA at
2,951 cm-1(corresponding to the symmetric stretching of the CH2bond). In Fig. 4
spectra of tested materials are reported. Using the spectral calculator, three
theoretical spectra were numerically evaluated and the peak intensities, corre-
sponding to the characteristic bands of the homopolymers, were calculated. From
these values the ratios (R) was calculated and a work line was constructed. From its
expression and the evaluation of the ratios for real synthesized copolymers, final
macromolecular compositions were obtained.
Obtained results from FT-IR analyses are reported in Table 7.
Experimental results showed that the real macromolecular composition evaluated
from FT-IR spectra of materials can be controlled adopting the seeded semi-batch
emulsion copolymerization scheme, because all synthesized materials showed a
final molar composition very similar to the global recipe composition.
Chemical analysis
1H NMR analysis was performed on polymeric solutions in CDCl3. Because of only
one difference occurred between the monomeric units (the alcoholic group bonded
to the carboxylic group), from obtained spectra was not possible to detect chemical
shifts due to this difference.
650850 10501250145016501850 205022502450 2650
Wavenumber [cm^-1]
285030503250 3450 36503850
Transmittance [%]
PMMA
P(MMA-co-BMA)
P(MMA-co-BMA) 75/25
P(MMA-co-BMA)
PBMA
Fig. 4 Spectra of the synthesized materials. In the spectra of the homopolymers are highlighted the
characteristic bands used to quantify the copolymer compositions
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From this analysis was in any case possible to deduce the stereochemical
configuration of the macromolecules that resulted atactic for all synthesized
materials.
Thermal analysis
Glass transition temperatures were evaluated for all synthesized materials. Obtained
results for copolymers were compared to homopolymer results. Samples underwent
two scans and Tgvalues were evaluated using the second scan thermograms.
For PMMA, the Tgwas 118 ?C while for PBMA was 24 ?C. Introduction of
PBMA units within the copolymer macromolecule increased the chain flexibility
and the Tgfor copolymers showed intermediate values. Results obtained were
reported in Table 8.
Experimental behaviour can be compared to theoretical approaches proposed by
Fox (Eq. 7) [30] and Gordon–Taylor (Eq. 8) [31] equations:
1
Tg;copolymer¼wMMA
Tg;MMAþwBMA
Tg;BMA
ð7Þ
Tg;copolymer¼wBMA? Tg;BMAþ k ? 1 ? wBMA
ð
ð
Þ ? Tg;MMA
Þ
wBMAþ k ? 1 ? wBMA
ð8Þ
where wMMAand wBMAare the weight fractions of the functional units present in the
copolymer chain, Tg,MMAand Tg,BMAare the glass transition temperatures of the
homopolymers and k is a characteristic coefficient of the material [32], defined as
q1
q2?Da2
expansion coefficient of the component i; k is generally replaced by the fitting
parameter C obtained from experimental data. Depending from C value, the curves
representing Tgversus the macromolecular composition deviate from the linear
behaviour, defined by C = 1.
Da1, where qiis the density and Dai= amelt– aglassis the increment at Tgof the
Table 7 Ratios between peak intensities of characteristic bands of PMMA and PBMA in copolymers
spectra, and percentage macromolecular weight and molar compositions of synthesized materials
Material
R
% weight composition
MMA/BMA
% molar composition
MMA/BMA
P(MMA-co-BMA) 87.5/12.50.538 82.4/17.685.9/14.4
P(MMA-co-BMA) 75/250.487 67.1/32.9 73.5/26.5
P(MMA-co-BMA) 50/500.40542.1/57.9 50.7/49.3
Table 8 Glass transition
temperatures evaluated for
tested materials
Materials
Tg(?C)
PMMA 118
P(MMA-co-BMA) 87.5/12.5 95
P(MMA-co-BMA) 75/25 80
P(MMA-co-BMA) 50/5055
PBMA 24
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Page 14

For the fitting of the experimental data it is convenient to linearize the Gordon–
Taylor equation as follows:
?
where the components A and B are assigned such Tg,A\Tg,B.
From experimental results a plot of (Tg,copolymer– Tg,PBMA) versus (Tg,PMMA–
Tg,copolymer)?wMMA/wBMAyields C from a linear interpolation, as reported in Fig. 5.
Slope of the linear interpolation represents C parameter.
The C value obtained (0.6453) was employed to determine the Gordon–Taylor
curve following reported. In Fig. 6, glass transition temperatures, evaluated
experimentally and using both Fox and Gordon–Taylor equations, are reported.
From theseresultsispossibletoobserveanegativedeviationoftheexperimental data
withrespectofthelinearbehaviour;also,awelldescriptionofexperimentaldatawith
Gordon–Taylor equation, that describes the dependence of the glass transition
temperature from the composition of random copolymers, can be highlighted.
The Fox equation represents a rough evaluation of the Tgfor the copolymers,
based on the assumption that the glass transition temperature of a copolymer is only
an additive combination of the Tgof the corresponding homopolymers but it is not
verified for many copolymers. The Gordon–Taylor equation takes also the C
parameter into account. This parameter is related to the densities and the coefficients
of expansion of the materials, and it represents a more accurate model to describe
the thermal behaviour of random copolymers. The correspondence of experimental
Tgwith the Gordon–Taylor model could be attributed to the greater accuracy of the
Gordon–Taylor equation with respect of the Fox model. The Fox equation in
general could be used to evaluate positive or negative deviations in thermal
behaviour with respect to the linear trend, instead, the Gordon–Taylor can be used to
carefully predict the Tg, when the C parameter is known, as in this work.
Tg? Tg;A¼ C ? Tg;B? Tg
??wB
wA
ð9Þ
y = 0,6453x
R2 = 0,9834
0
10
20
30
40
50
60
70
80
0 2244 66 88110
(Tg,mma-Tg,copolymer)*wmma/wbma [°C]
(Tg,copolymer-Tg,bma) [°C]
Fig. 5 Evaluation of the C parameter from the linear interpolation of the (Tg,copolymer– Tg,PBMA) values
versus (Tg,PMMA– Tg,copolymer)?wMMA/wBMAvalues
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Conclusions
In the present work an optimized method to synthesize copolymers with controlled
macromolecular composition was proposed. A specific monomer-seeded semi-batch
emulsion polymerization was carried out in order to obtain a poly(methyl
methacrylate-co-butyl methacrylate) copolymer with three different compositions.
This useful scheme of reaction was developed parcelling the feed in nine parts,
paying attention to the elapsing time between the feeds avoiding the drift of residual
unreacted monomer. From HPLC results (also confirmed by studies onto the latex)
was possible to conclude that monomers consumption was almost complete and the
final conversion was very high. The semi-batch reaction allowed controlling the
chain growth, kept down the dispersion indexes. FT-IR spectra of the synthesized
materials showed that the real macromolecular composition was well controlled
through the seeded semi-batch emulsion copolymerization scheme, being the final
molar composition very similar to the global recipe composition. Thermal analyses
showed that synthesized copolymers are well described by the theoretical Gordon–
Taylor model, with a negative deviation from the additivity of the glass transition
temperatures.
These controlled composition copolymers present high potentiality in several
engineering sectors, such as for example in biomedical engineering field, where a
P(MMA-co-BMA) material, having a specific macromolecular composition, could
find favourable use in ophthalmic application, orthopaedic sector (bone cement or
bone substitute) or in implantable devices as drug delivery platform.
0
20
40
60
80
100
120
0 1020 3040
% weight of BMA
50 6070 8090 100
Tg [°C]
Experimental
Gordon-Taylor eq.
Fox eq
Linear behaviour
Fig. 6 Glass transition temperatures evaluated experimentally (black diamond), via the Fox equation
(black square) and by the Gordon–Taylor equation (black circle)
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