Living Radical Polymerization - American Chemical Society

Chapter 20

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Acrylate Based Copolymers by ATRP: The Effect of Molecular Structure on ThermoMechanical Properties Kaloian Koynov1, Azhar Juhari1, Tadeusz Pakula1+, and Krzysztof Matyjaszewski2 1) Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany; 2) Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213

Acrylate copolymers with complex linear or star architectures were prepared and characterized. Precise control over the sequence distribution and overall composition of these materials was achieved by atom transfer radical polymerization. A strong correlation between the molecular structure and composition of the copolymers and their thermomechanical behavior was found. This provides a new way for creating advanced materials with tailored properties.

+

deceased 06.07.2005

Introduction With the development of living anionic polymerization (1,2) and controlled/living radical polymerizations (CRPs) (3,4) the synthesis of copolymers, especially with complex architecture, has recently received © 2009 American Chemical Society In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

297


298 increased attention due to interests in both the academia and industry. Atom transfer radical polymerization (ATRP) (5–8) is one of the most common CRP methods and has been widely used for the synthesis of (co)polymers with predetermined degree of polymerization, narrow molecular weight distribution (9), high functionality (10) and desired microstructure (11–16). The preservation of an active chain end, most frequently a halogen group, enables synthesis of di-, tri- or multi-block copolymers with various architectures, such as stars (17–21) and brushes (22-25). The complex molecular structure has major effect on the thermo-mechanical behavior of these materials and allows tailoring of their properties. In this contribution we show how to tune the thermo-mechanical properties of several acrylate based copolymers with complex architectures, synthesized via ATRP. First, we will discuss well-defined statistical, gradient and block copolymers consisting of isobornyl acrylate (IBA) and n-butyl acrylate (nBA). The thermo-mechanical properties of these materials were analyzed via differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA), tensile tests and small-angle X-ray scattering (SAXS). The glass transition temperature (Tg) of the statistical (P(IBA-co-nBA)) copolymers can be continuously tuned by changing the monomer feed. This way, it is possible to generate materials that can mimic thermal behavior of acrylate homopolymers but have significantly different mechanical properties. As a second example, the thermoplastic elastomer behaviour of tri-arm star block copolymers poly(n-butyl acrylate)-b-polyacrylonitrile (PBA-PAN) and poly(nbutylacrylate)-b-poly (methyl methacrylate) (PBA-PMMA) is presented. These materials retain phase separated morphology until very high temperature and their tensile properties can be easily tuned by adjusting the copolymers composition.

Linear copolymers of isobornyl acrylate and n-butyl acrylate The structure-property correlation of different copolymers with n-butyl acrylate and isobornyl acrylate units have been studied. The primary goal was to compare thermomechanical properties of block, gradient and statistical copolymers of nBA and IBA with various acrylate homopolymers (Scheme 1). The choice of nBA and IBA was dictated by very different thermal properties of the resulting homopolymers, glass transition temperature (Tg) of PnBA is -54°C while the Tg of PIBA is 94°C. Thus, their copolymerization with carefully selected ratios should result in polymers with thermal properties, i.e., Tg similar to acrylate homopolymers: poly(t-butyl acrylate) (PtBA), poly(methyl acrylate) (PMA), poly(ethyl acrylate) (PEA) and poly(n-propyl acrylate) (PPA).

In Controlled/Living Radical Polymerization: Progress in ATRP; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

299 Monomer Polymer n-butyl acrylate

R epeating unit CH 2

T g (o C ) -54

CH O C O n CH 2 CH 2 C H 2 C H 3

n-propyl acrylate

CH 2

CH O C O n CH 2 CH 2 C H 3

ethyl acrylate

CH 2

CH O C O CH 2

methyl acrylate

t-butyl acrylate

CH 2

CH 2


CH 2

random copolymers

n

PEA 10 n

block copolymers

PMA PtBA

43

CH

CH O C O

compare to

PPA

-24

CH 3

CH O C O CH 3

O C O H 3C C CH 3 CH 3

isobornyl acrylate

various IBA/nBA ratio -40

n

gradient copolymers 94 n

Scheme 1: Copolymerization of isobornyl acrylate and n-butyl acrylate in different fashion and comparison of properties of resulting copolymers with acrylate based homopolymers such as PtBA, PMA, PEA and PPA.

Despite similar Tg values, the mechanical properties of the copolymers and homopolymers can significantly differ, depending on the type of building units and their arrangement. In a similar way, one can copolymerize other monomers and affect not only physical properties of the final polymer materials, but also their degradation rates and toxicity, both of which are very important, for example, in biomedical applications. Statistical copolymers of nBA and IBA with different molecular weights and compositions were synthesized under ATRP conditions, as described in detail earlier (26). In all ATRP reactions, a CuBr/PMDETA complex was used since it is commercially available and well mediates controlled polymerization of acrylate monomers. Polymerizations were performed at 50°C in acetone/anisole mixture using EtBrIB as the initiator. The schematic representation of all prepared materials is shown in Scheme 2. The solid line represents a series of polymers with similar DP but systematically increasing IBA content. Another such group of copolymers is indicated with a dashed line. Copolymers with similar IBA/nBA ratio but different degree of polymerization (DP), i.e., the dotted line, were also synthesized. When comparing the thermo-mechanical properties of acrylate homopolymers and P(IBA-co-nBA) copolymers, the first important question is whether the copolymer system is isotropic in the bulk state or rather exhibits a micro-phase separation. To answer this question, the DSC thermograms for all samples shown in Scheme 2 were measured.


300 100

Weight % IBA

80 60 40 20 0 100

1000

Scheme 2: Schematic representation of prepared copolymer samples of P(IBAco-nBA) with different molecular weights, compositions and architectures: statistical (open squares), block (rhombi) and gradient (circles).

All statistical copolymers are completely amorphous and exhibit only a single glass transition at a temperature that increases with increasing IBA content. This indicates that there is no micro-phase separation in these materials, a finding further confirmed by DMA and SAXS experiments. The Tg’s of the statistical copolymers from the solid and dashed lines in Scheme 2 are plotted vs. their IBA content in Figure 1.

Tg predicted, Fox equation Tg measured (dotted line, Scheme 2) Tg measured (solid line, Scheme 2) Tg of homopolymers

100 80 60

PtBA

40

Tg [°C]


DP Monomer1+2

20

PMA

0 -20

PEA

-40

PPA

-60 0

10

20

30

40

50

60

70

80

90

100

Weight % of IBA

Figure 1: Tuning Tg in statistical copolymers of P(IBA-co-nBA) with different IBA/nBA ratios and two different degrees of polymerization (Scheme 2, copolymers on solid and dotted lines).


301 The Tg varies continuously with composition in both low and high molecular weight samples. To describe such type of composition dependence of the Tg of copolymers or miscible polymer blends, the so-called Fox equation (27) is often used:

Here Tg is the glass transition temperature of the copolymer, Tg1 and Tg2 are the glass transition temperatures of the two homopolymers, and w1 and w2 are the weight fractions of the two repeat units in the copolymer. As can be seen in Figure 1, the experimentally measured Tg values closely follow the predictions of the Fox equation, which is represented by the solid line. Figure 1 demonstrates that it is possible to generate statistical P(IBA-co-nBA) copolymers that can mimic the thermal behavior of several homopolymers, such PtBA, PMA, PEA, PPA. It is interesting to verify whether copolymers can mimic not only the Tg’s of the acrylate homopolymers, but also their mechanical properties. In order to do this, the dynamical mechanical analysis (DMA) was performed and the frequency dependencies of the storage (G’) and the loss (G’’) parts of the shear modulus (master curves) of copolymer/homopolymer pairs with similar molecular weight and glass transition temperatures were compared. Typical results from such comparison between PEA homopolymer (Tg = -24°C, Mn = 96900 g/mol) and IBA(33%)-nBA(67%) copolymer (Tg = -20°C, Mn = 107500 g/mol) are shown in Figure 2.

G'

G'' PEA IBA(33%)/nBA(67%)

7

10

G',G'' [Pa]


w w 1 = 1 + 2 T g Tg 1 T g 2

4

10

Tref=15°C 1

10

-5

10

-2

10

1

10

4

10

7

10

ω [rad/s]

Figure 2:. Comparison of the shear moduli spectra of PEA, (Tg = -24°C, Mn = 96900 g/mol) and IBA(33%)-nBA(67%) copolymer (Tg = -20°C, Mn = 107500 g/mol).

The two master curves overlap very well both in the high frequency region (due to the similar Tg’s) and in the low frequency region (due to the similar Mw



302 values). Nevertheless, a closer look reveals that in the characteristic rubbery plateau region the storage modulus (G’) of the P(IBA-co-nBA) copolymer is significantly lower than the corresponding value for the PEA homopolymer. This means that while mimicking perfectly the glass transition temperatures of the PEA, the P(IBA-co-nBA) copolymer is significantly softer than its homopolymer counterparts. The data for the G’ in rubbery plateau region for all copolymer/homopolymer pairs studied are summarized in Table 1. For comparison the values for the PnBA and PIBA homopolymers are also shown. As can be seen from the table in all cases, the P(IBA-co-nBA) copolymers are significantly softer than the respective homopolymers having similar glass transition temperatures. The lower plateau modulus of the copolymers indicates that they should have higher entanglement Mw values. The effect is most likely related to the rather big and rigid side group of IBA, which prevents the copolymers chains from a denser packing and therefore results in a less dense and softer IBA containing copolymers. This conclusion is also confirmed by the very low plateau modulus of the IBA homopolymer itself. Further detailed studies are required, however, to get better inside on the observed trends in the plateau modulus dependence on copolymer composition. Table 1: Comparison of the glass transition temperature and the rubbery plateau storage modulus of acrylate co- and homopolymers Polymer PPA IBA(16%)-nBA(84%) PEA IBA(33%)-nBA(67%) PtBA IBA(78%)-nBA(22%) PnBA PIBA

Tg [°C] -40 -39 -24 -20 37 42 -54 105

Plateau G’ [kPa] 140 95 170 70 135 85 130 80

Gradient and block copolymers of nBA and IBA. As discussed above, the statistical P(IBA-co-nBA) copolymers exhibit “homopolymer type” thermomechanical properties, with no evidence for a two phase behavior. However, the situation is remarkably different in the case of gradient and block copolymers (28-31). In order to illustrate this, the left panel of Figure 3 compares DSC and DMA spectra of a statistical and a gradient copolymer, with similar composition and molecular weight. The statistical copolymer shows a clear glass transition at a temperature between the Tg’s of the two homopolymers. The gradient copolymer, however, reveals a broad glass transition region with no obvious Tg and a very broad distribution of relaxation processes. The right panel of Figure 3, compares statistical and block copolymers. While, the statistical copolymer has a single glass transition, the block copolymer shows two glass transition temperatures corresponding to the Tg’s of each component indicating a microphase separation in the sample. With respect to the frequency dependence of G’ and G’’ it is important to emphasize that in the case of the block copolymer,


303

10

9

10

6

10

3

Tref=15°C

G'

G',G'' [Pa]

G',G'' [Pa]

merely an attempt for constructing a master curve is shown, as the temperature dependence of the shift factors does not conform to the WLF equation.

G'' statistical gradient

0

10

9

10

6

10

3

Tref=15°C

G'

statistical block 0

10 -7 10

10

-4

10

-1

10

2

10

5

10

10 -15 10

8

10

-10

ω [rad/s]

10

-5

10

0

10

5

10

10

ω [rad/s] statistical gradient

statistical gradient

-0.1

ΔH [a.u.]

-0.1

ΔH [a.u.]


G''

-0.2

-0.3

-0.2

-0.3 -60

-30

0

30

60

Temperature [°C]

90

120

-60

-30

0

30

60

90

120

Temperature [°C]

Figure 3: Comparisons between gradient IBA(47%)-nBA(53%), Mn = 74800 g/mol and statistical IBA(44%)-nBA(56%), Mn = 53000 g/mol copolymers (left panel) and block IBA(54%)-nBA(46%), Mn = 54600 g/mol and statistical IBA(54%)-nBA(46%) Mn = 69000 g/mol copolymers (right panel).

Nevertheless, this “master curve” provides additional evidences for the microphase separation in the sample. First of all the glass transitions of the two blocks are clearly distinguishable at low and high frequencies, respectively. Furthermore the behavior of the “master curve” in the very low frequency region, namely G’(ω) overlapping with G”(ω) and having a slope of ~ 0.5 may indicate the formation of a lamella structure as theoretically predicted (32).

Polar 3-arm Star Block Copolymer based on n-butyl acrylate Thermoplastic elastomers (TPE) are an important class of materials combining elastomeric behavior with thermoplastic properties. Commonly these are ABA type triblock copolymers such as polystyrene-polyisoprenepolystyrene (SIS) or polystyrene-polybutadiene-polystyrene (SBS) that combine a soft central block with glassy end blocks. Although these materials have found wide spread commercial applications, the presence of unsaturated double bonds in the middle segments makes them susceptible to oxidation and eventually reduce the use lifetime (33). Different strategies, including change of outer and inner segments of ABA triblock copolymers (34) or design of materials bearing



304 novel architectures and topologies (35-37) have been used to address this issue and prepare TPE with tailored properties. For example, during the last several years, it was demonstrated that thermoplastic elastomers based on star block copolymers (35,38,39) may exhibit superior properties, as compared to the linear ABA triblock copolymer type TPEs. It was reported that multi-armed polystyrene-b-polyisobutylene (PS-b-PIB) star block copolymers have substantially better mechanical properties and much lower sensitivity to diblock contamination than their linear triblock counterparts (39). With this in mind, triarm star block copolymer thermoplastic elastomers were prepared, which comprise of poly(n-butyl acrylate)-b-polyacrylonitrile (PBA-b-PAN) or poly(nbutylacrylate)-b-poly (methyl methacrylate) (PBA-PMMA) arms. The incorporation of the polar hard segment of PAN and PMMA was possible with the aid of halogen exchange ATRP technique (40-43). The effect of molecular architecture and composition on the thermo mechanical properties of these star block copolymers is discussed bellow. 3-arm star (PBA-b-PAN) block copolymers with different molecular weights and compositions were synthesized by ATRP using halogen exchange strategy, as described in detail elsewhere (42). The composition of the copolymers was determined by 1H NMR and is given in the Table 2. All copolymers had low polydispersities, as reported in Table 2.

Table 2. Series of 3-arm Star PBA-PAN block copolymers Sample T-PBA304-PAN63 T-PBA304-PAN109 T-PBA304-PAN207 T-PBA304-PAN331 T-PBA388-PAN115 T-PBA388-PAN165 T-PBA388-PAN264 T-PBA52-PAN26 T-PBA547-PAN153

Arm composition (BA304-b-AN63)3 (BA304-b-AN109)3 (BA304-b-AN207)3 (BA304-b-AN331)3 (BA388-b-AN115)3 (BA388-b-AN165)3 (BA388-b-AN264)3 (BA52-b-AN26)3 (BA547-b-AN153)3

Mn 127000 134000 150000 169000 167000 175000 191000 24100 234000

wt% PAN 8 13 22 31 11 15 22 17 10

Mw/Mn 1.14 1.17 1.17 1.11 1.07 1.13 1.15 1.22 1.16

Due to the strong incompatibility between the two phases, the PBA-PAN star copolymers are expected to show very strong phase separation. In an attempt to visualize this effect AFM imaging was performed on thin films prepared by spin-casting of polymer solutions in DMF onto silicon wafer substrates. Typical results are shown in Figure 4. The phase contrast AFM images of the films reveal the phase separated morphology of 3-arm star PBA-PAN block copolymers. Rigid PAN domains (brighter features in AFM images) are dispersed in the softer PBA matrix. The sample with lowest content of PAN (8wt%) shows round features, while the sample with 13wt% PAN displays a


305


mixture of round and elongated (cylindrical) features. As the round features may originate from either spheres or standing cylinders, other experimental techniques (e.g. SAXS as described below) are needed to unambiguously determine the film morphology. Finally the sample with highest PAN content (22wt%) displayed meandering curved features, characteristic for short-range ordered cylindrical structures or bicontinuous structures.

Figure 4: AFM phase images of 3-arm star PBA-PAN block copolymers: (left) T-PBA304-PAN63 (8wt% PAN); (middle) T-PBA304-PAN109 (13wt% PAN); and T-PBA304-PAN207 (22wt% PAN). Image size: 2 μm × 2 μm.

In order to get a better insight into the formed microstructures, SAXS analysis of thick films of all PBA-PAN copolymers listed in Table 2 was performed. Most of the materials revealed cylindrical morphologies and their different compositions affected the cylinders d-spacing in the micro-phase separated structures. Furthermore, the SAXS studies showed that annealing of the films for 2 hours at 150°C resulted in improved phase separation (42). Dynamic mechanical properties of the PBA-PAN star copolymers were characterized through the temperature dependencies of the real (G’) and imaginary (G”) parts of the complex shear modulus. Some typical results for a series of copolymers with constant PBA content (DP equal to 304) and varying PAN fractions are shown in Figure 5 together with the respective DSC thermographs. In every case, 4 major transitions were observed. The first two correspond to the glass transitions of PBA (~ -50 °C) and PAN (~ 105 °C). The third transition, at temperatures around 260°C, manifests itself in a rather sharp drop in the storage modulus G’. Finally, at higher temperatures (280-290°C) an exothermal process is observed in the DSC thermographs with intensity proportional to the content of PAN in the material.


306 10

11

G''[Pa]

10

10

7

10

5

10

3

10

8

10

6

10

4

10

2

T-PBA304-PAN207 T-PBA304-PAN109 T-PBA304-PAN63

4 3

-dH/dt


G'[Pa]

T-PBA304-PAN331 9

1

2

3

4

2 1 0 -100

0

100

200

Temperature [°C]

300

Figure 5: Thermomechanical and DSC analysis of 3-arm star PBA-b-PAN copolymers. This process is related to chemical cross-linking of the PAN segments. As for the mechanical properties, the samples were glassy below the glass transition temperature of PBA, with storage modulus G’~ 1 GPa. Above this glass transition, the copolymers become elastic and show a G´ plateau extending up to the softening temperature of PAN (~ 100 oC). The plateau value of G’ depends strongly on the PAN content: ~0.1 MPa for a pAN fraction < 0.1 and ~100 MPa for a PAN content of 0.3. In this elastic state the PAN blocks form the glassy domains connecting the flexible PBA blocks. This corresponds to a typical situation for a thermoplastic elastomer in which the hard phase elements (i.e. the glassy microdomains) act as physical crosslinks for the flexible component. Both storage and loss moduli decrease above the Tg of PAN but a second plateau with lower G’ values (0.1 – 1 MPa) extends until temperatures of ~260 °C. This indicates that the 3-arm star PBA-b-PAN copolymers maintain their microdomain morphology up to a very high ODT temperature of around 260°C, i.e., approximately 150°C above the glass transition of the PAN.


307

Stress [MPa]

20

T-PBA388-PAN115 T-PBA388-PAN165 T-PBA388-PAN264

15

10

5

0

1

2

3

4

5

6


draw ratio (λ)

Figure 6: Stress - strain curves recorded for PBA-PAN star copolymers with varying content of PAN for same length of PBA. Tensile mechanical properties of the PBA-PAN star copolymers were also investigated and some typical results are shown in Figure 6. The observed behavior is typical for elastomeric materials with elongations at break up to 600 %, and stress at break up to 13 MPa. As expected, the composition has a major effect on the tensile properties of the PBA-PAN star copolymers. The data showed that both the initial elastic modulus and the tensile strength increase strongly with PAN content, while the draw ratio at break seems to increase with the length of pBA segment and be reduced with increasing PAN content. This finding suggest an easy way for tuning the material properties by adjusting the copolymer composition. 3-arm star (PBA-b-PMMA) block copolymers have also been synthesized by ATRP (43). In order to study the effect of composition on the properties of the star PBA-PMMA block copolymers, 3 different samples with an increasing PMMA content were prepared. The precise compositions of these copolymers were determined by 1H NMR and are given Table 3. All materials exhibited good functionality and polydispersities below 1.3. As PBA and PMMA are strongly incompatible, the copolymers are expected to exhibit phase separated morphology. In order to study this effect in detail for a better understanding of the formed microstructures, SAXS analysis of thick films of the star copolymers listed in Table 3 was performed. Table 3. Series of 3-Arm Star PBA-PMMA block copolymers Sample T-PBA164-PMMA31 T-PBA164-PMMA76 T-PBA164-PMMA118

Arm composition ((BA)164(MMA)31)3 ((BA)164(MMA)76)3 ((BA)164(MMA)118)3

wt% of PMMA 13 26 36

Mw/Mn 1.30 1.29 1.28

First, as-cast samples were studied and found that only the copolymer with highest PMMA content showed a well-ordered structure, consistent with hexagonally packed cylinders. The samples with lower PMMA content exhibited short-range ordering only. In order to examine the effect of thermal treatment on the copolymer films morphology, the SAXS experiments were


308 repeated after annealing the PBA-PMMA samples at 150°C. The results are presented in Figure 7. 1000

1

100

1/2

3

10

1/2

7

1 0.02

0.04

0.06

0.08

0.10

0.12

-1

s [nm ]

Figure 7: SAXS of 3-arm stars PBA-PMMA block copolymers after thermal annealing. The annealed films exhibit good ordering, displaying at least three scattering peaks in each of the SAXS spectra. Furthermore, the ratios of the relative peak positions in all cases were s , 3s, 7 s , i.e., again consistent with a hexagonal lattice. It is important to emphasize that, while all three PBAPMMA copolymer studied exhibited similar cylindrical morphologies, their different compositions affect the cylinders d-spacing which increases with the increase of PMMA block length for the same length of the PBA segment. Tensile behavior is of primary importance when considering the properties of a thermoplastic elastomer. The stress–strain dependencies for the 3-arm star PBA-PMMA block copolymers listed in Table 3 are shown in Figure 8. The data were recorded at room temperature and with drawing rate of 5 mm/min. 12

stress σ [MPa]


Intensity [a.u.]

T-PBA164-PMMA118 T-PBA164-PMMA76 T-PBA164-PMMA31

36% PMMA 26% PMMA 13% PMMA

9

6

3

0

1

2

3

4

5

draw ratio (λ)

Figure 8: Stress-strain curves recorded for the 3-arm stars PBA-PMMA block copolymers.


309 The plots clearly indicate the elastomeric behavior of the star copolymers. The films of all three materials exhibited similar high elongation at break, about 500%, independent of the copolymer composition. As expected, the increase of the PMMA content in the copolymers resulted in an increase of the E modulus of the films from 1.02 to 4.76 and 11.18 MPa for samples with 13%, 26% and 36% PMMA respectively, suggesting a simple approach for tuning the tensile properties by adjusting the copolymers composition.


Conclusions Various acrylate copolymers with complex architectures have beensynthesized via atom transfer radical polymerization. A strong correlation between the molecular structure and composition of the copolymers and their thermo-mechanical behavior have been found. This suggests a facile route for creating new advanced materials with tailored properties. Acknowledgments: Financial support from NSF (DMR 05-49353), Universiti Teknologi MARA and the Ministry of Higher Education of Malaysia is gratefully acknowledged.

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Living Radical Polymerization - American Chemical Society

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