MORPHOLOGICAL CONTROL AND APPLICATIONS OF STEREOREGULAR SEMICRYSTALLINE MULTIBLOCK COPOLYMERS OF STYRENE AND BUTADIENE. Alfonso Grassi, Antonio Buonerba, Cinzia Cuomo and Francesco Pastore Università di Salerno, Dipartimento di Chimica, Fisciano (SA), Italy.
[email protected] Introduction Styrene-b-butadiene diblock polymers were synthesized since the late 50s by controlled/living anionic polymerization catalyzed by alkyl-lithium compounds or controlled radical polymerization.1 These copolymers are completely amorphous and were defined thermoplastic elastomer on the basis of the well established morphology and physico-chemical properties. Recently multiblock styrene-butadiene copolymers (sPS-B) comprising segments of syndiotactic polystyrene (sPS) and of cis-1,4 polybutadiene (PB) have been obtained with tuneable block lengths using monocyclopentadienyl titanium compounds activated with MAO.2-5 Crystallinity attributed to the syndiotactic polystyrene domains was found in the copolymer samples with styrene molar fraction xS > 0.37 and styrene average block length ns of at least eight monomer units. The CP MAS 13C NMR and WAXS analysis of the crystalline phase showed that it is in the crystalline form known for syndiotactic polystyrene. Experimental Copolymers sPS-B and Blends sPS-PB. The copolymers sPS-B, the blends sPS/PB, the blends sPS-B/PB and the blends sPS/sPS-B/PB were in situ synthesized and characterized according to previously reported procedures.2-5 Atomic Force Microscopy. AFM images of spin-coated polymer films were collected in tapping mode using a Nanoscope Dimension 3100 from Digital Instruments. 100 L of a chloroform solution of the polymer (0.2 wt %) were spin-coated on a glass surface for 15-20 s at a speed of 2000 rpm. To identify the rubber domain the film was exposed to OsO4 vapours (1 wt % water solution) for 25 minutes and then scanned. Mechanical Properties Measurement (Tensile tests). Stress-strain measurements were carried out on strips die-cut from pressed sheets (width, 6 mm; average thickness, 1 mm) by means of an Instron Series IX 4301 tensile testing dynamometer. 15g of polymer sample were dissolved in chloroform (1L) containing the crosslinking agent dicumylperoxide (2phr) and the antioxidant Wingstay K (0.5phr). The solvent was distilled off and the sample dried in a vacuo until a constant weight is reach. Then it was pressed at room temperature in a square frame, cured at 180°C in oven amid two steel plates protected by PTFE sheets and quenched in cold water. Results and Discussion Morphology of sPS-B copolymers. Samples 1-10 (Table 1) of sPS-B copolymers containing a range of styrene molar fraction of 0,40-0,98 were synthesized using the previously reported procedures. The samples with high styrene mole fraction (0,76-0,98) exhibit crystallinity of about 30%, melting point of about 250°C, Tg of 95°C and -85°C for the polystyrene for the polybutadiene phases, respectively. The former is that expected for polystyrene whereas the latter is higher than pure PB because of the incorporation of isolated styrene and 1,2vynyl butadiene units. 13C NMR analysis permitted full characterization of the monomer composition and polymer sequence.2 AFM images of film by spin coating of chloroform solution (0,2%w/w) of sPS-B showed a phase separated morphology in which spheres of PB of dimension of 50nm were detected in the hard phase of crystalline syndiotactic polystyrene (Figure 1).
Table 1. Multiblock copolymers sPS-B investigated. Hm of sPS Crystallinity (%) (J/g)
Sample
xS
Tm (°C)
1
0.40
244
11
22
2
0.42
235
12
23
3
0.57
241
12
23
4
0.68
228
6
11
5
0.68
247
9
17
6
0.74
242
9
18
7
0.76
251
15
29
8
0.77
253
15
29
9
0.95
250
16
31
10
0.98
252
19
37
This morphology is similar to that typically observed for diblock styrene-butadiene copolymer of variable block lengths and average molecular 6 weight.
Figure 1. 3D Tapping mode AFM height image (on the left) and phase image (on the right) of the sample 8 (scan size: 1μm; z-scale: 20 nm). When the styrene concentration was decreased to 0.4 typically jagged lamellas were observed (Figure 2). Amorphous materials were obtained with styrene molar fraction lower than 0,4 in which morphological control is missing because of random distribution of styrene in the PB chain.
Figure 2. 3D Tapping mode AFM height image (on the left) and phase image (on the right) of the sample 2 (scan size: 600nm; z-scale: 200 nm). Aiming to obtain polymer films containing isolated domains of crystalline sPS in PB matrix, blends of PB and sPS-B were synthesized using a multistep in situ polymerization process. Styrene and butadiene were first copolymerized to lead to the copolymer with the desired block length and crystallinity and the reactor was then charged with butadiene in large excess to produce the PB matrix. Samples with xS of 0,33 styrene yielded to polymer samples in which lamellae of sPS were detected in the by AFM analysis.
Polymer Preprints 2010, 51(1), 169
Proceedings Published 2010 by the American Chemical Society
Styrene (wt % )
Young’s Modulus (MPa)
Toughness (MPa)
Hm (J/g)
Crystallinity (%)
Table 2. Mechanical Properties (Derived from Stress-Strain Curves a) for sPS-PB, sPSB-PB and sPS-sPSB-PB blends after curing at 180°C for 35 min.
11
7
4.1±0.6
0.2±0.1
-
-
12
15
11±1
2.0±0.5
18
34
13
34
101±6
4.1±0.4
21
39
Sample
Figure 3. 3D Tapping mode AFM height image (on the left) and phase image (on the right) of sample 17 (scan size: 2m; z-scale: 300 nm). Use of sPS-B as compatibilizer of blends sPS-PB. The sPS-B copolymer were tested as compatibilizer of blends of sPS and PB (sPS-PB). The blends 18-19 were synthesized by polymerizing first styrene to yield the sPS fraction. Then butadiene was admitted into the reactor in the appropriate concentration to produce the copolymer with the expected composition and properties. Finally the reactor was charged with butadiene to yield the desired fraction of PB. The mechanical properties of the samples 18-19 were compared with those of blends of sPS and PB (sPS-PB, sample 11-14), of blends of sPS-B and PB (sPS80-B-PB, samples 15-17), of blends of sPS-PB compounded with sPS-B (c-sPS-PB-sPS80-B, samples 20-21). The stress strain curves showed enhanced mechanical properties for the in situ synthesized blends of sPS and PB compatibilized with the sPS-B copolymer (Table 2). These data can be interpreted in the light of morphology and crystallinity of the samples, the latter determined by DSC analysis by comparing the H of melting of the sPS crystallites with that of pure sPS. The blends containing 17 and 39% w/w of sPS compatibilized with sPS-B copolymers with 80w/w of styrene (sPS80-B) and styrene block length of 20 units exhibit crystallinity value lower than the in situ blends of sPS and PB with the same composition, suggesting a good dispersion of small crystallites of sPS in the rubber phase. AFM analysis showed in this case the presence of sPS particles of 100nm surrounded by a sphere of sPS-B. Interestingly two melting temperature were found at 198 and 260°C in the DSC curve of sample 8 (Table 2). The latter is attributed to the melting of sPS whereas the former is attributed at the sPS-B interphase between the sPS particles and the PB matrix. Actually the Figure 4 shows as the copolymer sPS-B lies at the interface between the sPS particles of average value of 100nm and the PB matrix.
sPS-PB
sPS80Bb PB
14
46
164±18
0.2±0.1
20
38
15
16
21±4
0.3±0.1
-
-
16
34
29±3
0.8±0.2
-
-
17
49
59±6
0.9±0.2
4
8
sPSsPSBPB c-sPSPBc sPS80B
18
15
24±1
1.5±0.2
30
57
19
24
31±1
2.5±0.9
35
65
20
17
13±1
3.9±0.2
16
31
21
39
175±5
1.3±0.1
20
38
PB
22
-
3.4±0.1
0.4±0.2
-
-
a
Average values of at least three measurements. b sPS80B indicates a copolymer containing 80wt% of styrene. c c-sPS-PB- sPS80B indicates blends obtained by compounding in chloroform the blends sPs-PB with a copolymer sPS80B. Acknowledgements Financial support of the “Ministerodell’Istruzione, dell’Università edella Ricerca” (PRIN2007) is gratefully acknowledged. The authors are also grateful to Dr. Vito Speranza, Dr. Fabrizio Bobba, Dr. Alessandro Scarfato for support in AFM analysis. References (1.) Hsieh, H. L.; Quirk, R. P. Anionic Polymerization principles and practical applications; Marcel Dekker, Inc.: New York, 1996. (2.) Grassi, A.; Caprio, M.; Zambelli, A.; Bowen, D. E. Macromolecules. 2000, 33, 8130-8135. (3.) Zambelli, A.; Caprio, M.; Grassi, A.; Bowen, D. E. Macromol. Chem. Phys. 2000, 201, 393–400. (4.) Caprio, M.; Serra, M. C. Bowen, D. E.; Grassi, A. Macromolecules 2002, 35, 9315-9322. (5.) Cuomo, C.; Serra, M. C.; Gonzalez Maupoey, M.; Grassi, A. Macromolecules 2007, 40, 7089-7097. (6.) Kirk-Othmer. Encyclopedia of Chemical Technology. 4th Ed. Wiley. Vol.21.
Figure 4. Tapping mode AFM (height image) of the in situ synthesized blend sPS/sPS-B/cis-1,4-PB (sample 18) containing totally 15 wt% in styrene (scan size: 1μm; z-scale: 80 nm).
Polymer Preprints 2010, 51(1), 170
Proceedings Published 2010 by the American Chemical Society
