Macromolecules 1990,23, 540-548
540
Molecular Structures in Isotactic Polystyrene Thermoreversible Gels Marc Klein,+Annie BrQlet,*and Jean-Michel Guenet' Laboratoire de SpectromBtrie et d'lmagerie Ultrasonores, Universit6 Louis Pasteur, CNRS U A 851,4, rue Blaise Pascal, F-67070 Strasbourg Cedex, France. Received March 1, 1989; Revised Manuscript Received J u n e 7, 1989
ABSTRACT: The chain helical structure, the chain-solvent arrangement, and the chain trajectory are investigated in isotactic polystyrene (ips) thermoreversible gels by three techniques: neutron diffraction, swelling experiments, and small-angle neutron scattering. From the neutron diffraction experiments on gels prepared from various solvents, it is again concluded that the 12, helical form is absent in the nascent state (undried and unstretched). As already proposed in a previous paper, a solvated near-3, helix seems to be a better candidate to account for the results (ladderlike model). The swelling experiments support this model by indicating that the gel behaves as a polymer-solvent compound whose stoichiometry is defined a t C, = 0.3 in cis-decalin and C, = 0.38 in trans-decalin, values in good agreement with those found previously by calorimetry and solvent crystallization. The small-angle neutron scattering on iPS/cis-decalin gels reveals a most unusual behavior in that the chains become stiffer and stiffer up to C = 0.3 (increase of the chain radius of gyration and q-' behavior consistent with rodlike portions larger than 300 A) then lose their stiffness (decrease of the chain radius of gyration and loss of the q-l behavior). Such behavior is in accord with the temperature-concentration phase diagram established earlier. All the results are discussed in the light of various molecular models.
Introduction Investigations on the molecular structures involved in iPS thermoreversible gels have provided one with numerous results that have raised several questions. Different answers have been put forward, leading eventually to conflicting Girolamo et ala,' after discovering by X-ray diffraction a new reflection at 0.51 nm on partially dried and stretched gels, first inferred that gelation might arise from stereoregularity defects on the chain. Later they accounted for the 0.51 reflection through the existence of a new helical form: 12, with a 6-fold symmetry.' To explain why a new helical form could be formed in gels rather than the usual 3, helical structure, Sundararajan et al.3 proposed this form to be stabilized by solvent molecules, that is solvated helices and correspondingly solvated crystals. More recently, Guenet4 showed by means of neutron diffraction experiments, for which only the polymer was labeled, that the 0.51-nm reflection was in fact not characteristic of the polymer but could be found in liquid decalin. From simple geometrical argument, Guenet, while retaining the idea of helix solvation, considered a near3, form.4 The nascent gel diffraction pattern, which is reminiscent of liquids, led to a structure (ladderlike model) closer to nematic liquid-crystalline than crystalline polymers. From neutron scattering experiments intended to determine the chain conformation, it was found that the chains adopt a wormlike s t r u c t u r e with a statistical element of about 8 nm.6 This structure is similar to what is known for liquid-crystalline polymers and gives further credit to the ladderlike model. More recently, however, the helix solvation has been questioned on the basis of NMR experiment^.^ In this paper, we further test these models through three types of experiments: (i) neutron diffraction experiments on other solvents known to produce physical gels to discover whether the 0.51-nm reflection is also absent; Institut Charles Sadron, F-67083 Strasbourg Cedex, France. LBon Brillouin C E N Saclay, F-91191 GIF/ YVETTE Cedex, France.
* Laboratoire
0024-9297/90/2223-0540$02.50/0
(ii) swelling experiments; (iii) neutron scattering investigation on the chain conformation as a function of polymer concentration in relation with the temperatureconcentration phase diagrams established previ~usly.~
Experimental Section (A) Material. The isotactic polystyrene (ips) samples were all synthesized following the Natta methods and are over 99% isota~tic.~ Two hydrogenated samples were used: iPSK, M, = 3.2 X lo5, MJM, = 2.8; iPSG, M, = 1.38 X lo5, M,/M, = 1.2. The characteristics of the deuterated samples are given in Table I. High-purity grade protonated cis-decalin was purchased from Merck Laboratories, while the deuterated cis-decalin was obtained from the Janssen company and is deuterated over 99%. All the solvents were employed without further purification. (B) Sample Preparation. A mixture of polymer and solvent was heated up to 175 "C in a test tube. Once a clear solution was obtained the gels were prepared by either quenching a t 0 "C and keeping the sample at this temperature for 30 min or quenching a t 14 "C and keeping the sample at this temperature for 2 h. For the neutron scattering experiments the solutions were cast in a mold described elsewhere: allowing the preparation of disk-shaped samples of 1-mm thickness and 1.5cm diameter. For the neutron diffraction experiments the gels were prepared in NMR silica tubes of 4-mm diameter sealed from the atmosphere. For the swelling experiments, cylindershaped samples of 1-cm height and 1-cm diameter were cut off from a gel slab obtained in the same type of mold as that employed for preparing the neutron scattering samples. (C) Neutron Scattering. 1. Setup. The experiments were performed a t the high-flux reactor of the Institute Laue-Langevin (ILL, Grenoble, France) on D11 and D17 small-angle cameras. The transfer momentum q = 4~/A[sin8/21 (A = neutron wavelength and 6 the scattering angle) was in the range 5 X 5 q (A-') 5 1.5 X for D11 (A = 10 A) and IO-' < q (A-') 5 1.3 X lo-' for D17 (A = 12 A). A mechanical wavelength selector was used on both apparatus, providing a wavelength distribution characterized by &A/ h
= 9%.
The gels were held in quartz containers (22-mm outer diameter, 19-mm inner diameter, and 1-mm thickness). The experiments were carried out a t 18 f 1 "C. The experiments on the all-labeled samples were carried out a t the Laboratoire LBon Brillouin (CEA-CNRS) a t Saclay on
0 1990 American Chemical Society
Macromolecules, Vol. 23, No. 2, 1990
Molecular Structures in Polystyrene Gels 541
Table I Molecular Weights of the Deuterated Samples from GPC in THF at 20 OC and from Small-AngleNeutron Scattering (SANS) on Gels Prepared at 0 O c a MW MW sample MwGw MWIMnGw (22.8P) (30w D0.74 7.4 X lo4 1.1 6 X lo4 5.6 X lo4, 5.4x 104 a D1.5 1.5X lo6 1.13 1.2 X 10' 9 X lo4, 1.1 x 106" D2.5 2.5 X 10' 1.15 2.2 X 10' 2.2 X lo6, 2.2 x 105 D0.6 6.0X lo4 1.1
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The overall polymer concentration is given in parentheses. Gels prepared at 14 'C. PAXY. A mechanical wavelength selector was also used, providing a wavelength distribution characterized by Ah/X = 8%. With h = 5 A the transfer momentum was in the range 1.5 X lo-' Iq (A-') I 1.3 X 10-l. 2. Signal Processing. In order to measure the coherent scattering from the deuterated chains only, a mixture of D + H decalin was used (21.3% D decalin + 78.7% H decalin), the scattering of which matches that of protonated polystyrene? The scattered intensities were corrected for sample transmission. As usual, a blank signal, scattered by a mixture of hydrogenated polymer and a blend of deuterated and hydrogenated solvent (same composition as above) was removed from the total scattered intensity to correct for incoherent scattering. Finally, the spectra were normalized by an incoherent spectrum scattered by protonated water. The normalized intensity then reads
IN(d = ~ ~ d M w s , ( q ) (1) where C, is the concentrationof labeled material,M, the weightaverage molecular weight, and S,(q) the scattering function of the deuterated chains. K is a constant, which presently is given as K = [4*6N*(aH-a,)21/[M,2(1- r ) ]
I
,
(2)
where 6 is the water-samplethickness (in cm),N A is Avogadro's number, M D is the monomer molecular weight, T is the water transmission, and aH and U, are the scattering lengths (in barns) of the deuterated species and of the mixture iPSH + D decalin + H decalin, respectively. (D) Neutron Diffraction. The experiments were performed at the ORPHEE reactor located at the Laboratoire Leon Brillouin (LLB, CEN Saclay, France) on the spectrometer 7C2 equipped with a banana-type detector made up of 640 cells with an angular resolution of 0.2O. We used cold neutrons with a wavelength of 1.095 A obtained from reflection of the primary beam onto the (111) plane of a Germanium crystal. The scattering vectors available ranged from 0.25 to 10 A-'. The signal was normalized by both a vanadium sample and a Plexiglas sample. Further details on the apparatus are available on request. (E) Swelling Measurements. Appropriate samples were immersed in an excess of solvent. The swelling degree G was taken as the ratio of the sample weight after a given time t over the sample weight prior to immersion. The equilibrium swelling degree G, was determined when the sample no longer swelled. All the experiments were carried out at room temperature.
Results and Discussion (A) Neutron Diffraction. In a previous paper,4 it had been observed f r o q neutron diffraction experiments carried out on iPS/cis-decalin gels that the 0.51nm reflection observed by X-ray diffraction on these gels and assigned to a new helical form (12, helix) was not found in the nascent gel. The fact reported by Keller et a1.l' that a thorough removal of the preparation solvent usually leads to the disappearance of this reflection is in agreement with these conclusions. Yet, for one solvent, i.e., trans-decalin, supposedly decalin-free samples are
Figure 1. Neutron diffraction patterns for deuterated (a) and hydrogenated cis-decalin (b). Intensity in arbitrary units.
reported to still exhibit this reflection when examined by X-ray diffraction." As a result, the question still holds as to whether the 0.51-nm reflection could also be present in the nascent gel state, undried and unstretched. To answer these questions and to supplement previous findings, we have carried out a series of neutron diffraction experiments on iPS/trans-decalin gels as well as on other systems. 1. Principle of the Experiments. The use of either hydrogenated or deuterated samples allows one to modify the diffraction power of the constituents of a system in a neutron diffraction experiment." Figure 1 shows the attenuation of the first diffraction maximum when hydrogenated decalin is used in lieu of deuterated decalin. This property is particularly interesting when polymers composed of hydrogen and carbon are dealt with. If all the species have the same type of labeling, either deuterated or hydrogenated, a neutron diffraction experiment yields results quite similar to those obtained by X-ray diffraction. Conversely, if one component is deuterated, the other one being still protonated, then the diffraction power of the former will be enhanced by at least a factor of 4 (bD2/bH2 = 4). Making use of this property, the polymer structure can be investigated by using a deuterated polymer and a hydrogenated solvent. As a result, the drying process is no longer necessary. The intensity is given as I ( q ) = A,2(q)Sp"oh(a)+ A,2(q)s,"h(q) + 2A,(q)A,(q)s,,""h(q)
+ spine + SshC(3)
where A, and A , are the structure factor of the solvent and the monomer, respectively, Scohthe coherent scattering functions, and s'""the incoherent scattering. The deuterated polymer incoherent signal is usually very small and thereby neglected. While the solvent intensity is strongly attenuated, it is not negligible and must be accordingly removed. A measure on the only solvent gives I,(q) = A,2(q)Ssmh(q)+ SShC (4) Once this intensity is properly substracted from (3), I ( q ) is (5) I ( q ) = A,2STh(q) + 2A,(q)Ap(q)s,,""h(q) This is almost the intensity diffracted by the polymer, except for the cross-term. This term is usually weaker and should not have any bearing in the present case. As detailed above, 7C2 is a high-resolution spectrometer. In order to detect with certainty the weakest reflections, the number of counts/cell was never lower than N = lo5. This means that the statistical dispersion N1/'/ N was lower than 0.3% .
542 Klein et al.
Macromolecules, Vol. 23, No. 2, 1990
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Figure 2. Neutron diffraction patterns of (a) iPSD/transdecalinH gel (D2.5, C = 37%), (b) liquid protonated transdecalin, and (c) the digrence of (b) from (a). Intensity in arbitrary units.
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Figure 3. Schematic representation of the ladderlike model. The phenyl rings are purposely oriented at random on the CC axis to emphasize that the chain conformation is not strictly a 3, helix. The hatched circles stand for the solvent molecules. Here too, the hatching is at random to illustrate the absence of orientation of the solvent molecules.
2. Results. Results obtained on iPSD/trans-decalin gels are reported in Figure 2. A maximum is seen a t 0.52 nm (Figure 2a). Yet, trans-decalin in the liquid state exhibits a maximum at 0.54 nm (Figure 2b). Once the trans-decalin signal is substracted from that of the iPSD/ trans-decalin gel, the intensity displays a broad maximum at 0.46 nm (Figure 2c). There is no detectable sharp peak at 0.51 nm that would reveal the presence of 12, helices. As in a previous publication4 we accordingly conclude that this helix is not characteristic of the gel structure in the nascent state. As was emphasized elsewhere, the first maximum for a liquid is related to the distance between first neighbors.12 This distance d can be estimated from
1.23X = 2d sin 012 (6) As with cis-decalin, for trans-decalin we end up with d = 0.66 nm. This value is very close to the pitch of the 3, helix (0.665 nm13). We thus conclude again that the polymer adopts a conformation close to the 3, helix, yet not regularly arranged as in crystals (see Figure 3). If the chain are under a near-3, conformation, then the solvent first-neighbor molecules can be in close contact, allowing maximum space filling. This model, designated as a ladderlike model, is similar to a molecular model proposed by Watanebe et al.,14 where the incorporation of solvent molecules into the a-helical form of poly(y-methyl L-glutamate) is considered. We have further examined other systems that are known to produce physical gels (Figure 4). iPSD/ 1-Chlorodecane a n d iPSD/Hexahydroindan. The diffraction pattern of the iPSD/l-chlorodecane gel (Figure 4a) does not reveal any sharp peak or maximum at 0.51 nm, even before solvent signal substraction. For the iPSD/hexahydroindan gel (Figure 4c), one perceives that the solvent is also responsible for the maximum a t 0.52 nm.
Figure 4. Neutron diffraction patterns for (a) iPSD/lchlorodecaneH gel (D0.6, C, = 49%), (b) liquid 1-chlorodecaneH, and (c) iPSD/hexahydroindanH gel (D2.5, Cpol = 28%). Intensity in arbitrary units.
Thus we have now studied four systems by the neutron diffraction technique together with the deuterium labeling method. In all cases no sharp reflection near 0.51 nm has been observed. Here we want to emphasize that we do not conclude that 12, does not exist. There are numerous evidences by Keller et a1.2 in favor of it. It would, however, be of interest to discover the growth mechanism of this helix if it is not present in the gel in the first place. (B) Gel Swelling. The swelling experiments consist of immersing a piece of gel in an excess of preparation solvent. An equilibrium swelling ratio G, is defined as
G, = Pm/Pg (7) where Po is the weight of the gel sample as-prepared and P, its weight after reaching equilibrium. Provided the piece of gel absorbs solvent without losing any polymer chains, an equilibrium concentration Cequ can be defined as a function of the preparation concentration Cprep: = Gm-lCprep (8) Results (Cequversus Cpre ) are given in parts a (cis-
decalin) and b (trans-decafin) of Figure 5. As can be seen, two regimes are present in both systems: for C < C,, there is no significant swelling and Cequ= Cprep;for C > C,, the gels always swell to the same equilibrium concentration independent of the preparation concentration Cprep. Further, the value determined for this equilibrium concentration is C, = 0.28 in cis-decalin and C, = 0.38 in trans-decalin. These results lead one to the following comments: (i) The swelling behavior is typical of and indeed very close to what one would theoretically expect for polymer-solvent compounds. As a matter of fact, as long as the stoichiometry is fulfilled, that is for Cprep< C,, there is no need for the system to absorb solvent unless there were a large amount of “amorphous” domains. Conversely, for C,, > C,, the system tends to reach stoichiometric conditions, hence solvent absorption takes place. Accordingly, the theoretical behavior for a perfect polymer-solvent compound will be Cequ= Cprepfor C < Cy and Cequ= C, for C > C,. For systems made up with solvent-free crystalline regions (no polymer-solvent compound formed) alternating with amorphous regions that are liable to absorb solvent, G, reads15 Gcs = 1 + (Ca/C-,- l)Cprep/Ca
(9)
where C, is the polymer-rich phase initial concentration and Cy its concentration after swelling to equilibrium.
Macromolecules, Vol. 23, No. 2, 1990
Molecular Structures in Polystyrene Gels 543 Table I1 Values of the Radius of Gyration Rg(A) as a Function of Gel Concentration.
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Figure 5. Equilibrium concentration Cgquversus preparation concentration Cprepfor (a) iPS/cis-decalin gels [(a)iPSK, (0) iPSG], (b) iPSK/trans-decalingels, and (c) a copolymerltransdecalin gel (from ref 15). In all cases the dotted line stands for Cequ = Cprep. The behavior described by relation 9 has already been observed with gels of multiblock copolymers (Figure 5c) for which it is known that the crystalline regions are solvent free.15 Unlike the polymer-solvent compound, the swelling behavior exhibits here an increasing departure from the law Cequ= Cprepwith increasing concentration. Further, no asymptotic behavior is reached. (ii) The value of C, determined by means of this method is close to those found from the gel formation and/or the gel melting enthalpies' !Cy = 0.28-0.3 in cis-decalin and C, == 0.4 in trans-decalin) or from the solvent crystallization method4 (C, = 0.28 in cis-decalin and C, = 0.4 in trans-decalin). (C) Neutron Scattering. 1. A Few Chains Are Labeled. The excess, normalized scattered intensity is expressed in the Zimm approximation:16 IN(q) = KC&fw[P(q) - 2AzC&Yq)1 (10) where C,, M,, and K have the same meaning as in ( l ) , P(q) is the chain form factor, and A, is the virial second coefficient. In the Guinier domain (qR < I), I,(q) reduces to IN(q) = KC&fw[l - q2(Rg)2/31[1- 2A,C,] Usually, the form C,JN-'(q) is preferred:
(11)
KCdNN-'(q)= [ l / M w ( l 4- q2(Rg)2/3)1i-2 A 2 c ~(12) In this representation, linearity can be experimentally obtained for qR > 1. For instance, R, for a Gaussian
Cpol~lo)
chain is measured to within 10% up to qR = 3. However, for highly branched polymers or compact structure the departure from linearity is already seen for qR < 1, which entails that the condition qR
