Interpenetrating Polymer Networks Based on Polybutadiene and

10 Interpenetrating Polymer Networks Based on Polybutadiene and Polystyrene Morphology and Phase Dimensions by Small-Angle Neutron Scattering and Electron Microscopy Downloaded by UNIV LAVAL on October 19, 2015 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch010

A. M . FERNANDEZ1, G . D .

WIGNALL1,2,

and L . H .

SPERLING1,2,3

Polymer Science and Engineering Program, Materials Research Center No. 32, Lehigh University, Bethlehem, PA 18015 2 Oak Ridge National Laboratory, Oak Ridge, T N 37830 3 Department of Chemical Engineering, Materials Research Center No. 32, Lehigh University, Bethlehem, PA 18015 1

Poly(cross-butadiene)-inter-poly(cross-styrene) interpenetrating polymer networks (IPNs), semi-IPNs, and chemical blends were prepared in which the second polymer synthesized, polystyrene, was fully deuterated to produce contrast for small-angle neutron scattering (SANS) and permit the dimensions of the individual polymer domains to be evaluated. Correlation lengths of 35-60 Åwere found for the IPNs, 50-100 Åfor the semi-I IPNs, and 160-80 Åfor the semi-II IPNs and the chemical blends; the larger correlation lengths corresponded to the lower level of cross-linking. Equivalent diameters were several hundred angstroms for the IPNs. Specific surface areas ranged from 200 to 20 m2/g for these same materials, in the range of true colloids. The morphology by transmission electron microscopy (TEM) is highly suggestive of dual phase continuity, especially for the full IPNs. The Debye theory SANS results correspond to the lower range of the TEM diameters. The two techniques have different systematic errors and measure somewhat different aspects of the morphology; therefore, comparisons are useful.

T H E ENGINEERING AND TECHNOLOGICAL USE of multicomponent polymeric materials occurs almost exclusively in the solid state. T o efficiently utilize these polymeric materials, a detailed understanding of their morphological structure is required. Small-angle neutron scattering (SANS) instrumentation can be applied to problems related to phase domain size and shape and 0065-2393/86/0211/0153$06.00/0 © 1986 American Chemical Society

In Multicomponent Polymer Materials; Paul, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

Downloaded by UNIV LAVAL on October 19, 2015 | http://pubs.acs.org Publication Date: December 9, 1985 | doi: 10.1021/ba-1986-0211.ch010

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MULTICOMPONENT POLYMER MATERIALS

interfacial areas. This method of analysis takes advantage of the contrast between the protonated (normal) and the deuterated molecules resulting from the differences in the coherent scattering length of deuterium and hydrogen atoms. Interpenetrating polymer networks (IPNs) are a special kind of multicomponent polymeric material. A n IPN may be defined as a combination of two polymers, each in network form, in which at least one was polymerized or cross-linked in the immediate presence of the other (1-3). Sequential IPN formation involves the synthesis of network I, a subsequent swelling of monomer II with initiator and cross-linking agent, and polymerization of monomer II in situ. In semi-I IPN materials, the first polymer synthesized is cross-linked, and the second polymer is linear. In semi-II IPN materials, the first polymer synthesized is linear, and the second polymer is cross-linked (2). Generally the IPN polymer components are structurally different, and immiscibility results from the low entropy of mixing. However, because of their interlocking chain structure, the extent of phase separation is restricted, and the domain dimensions are substantially smaller than those formed i n polymer blends (1-3). The morphology problems of IPNs, especially the sizes and shapes of the phase domains and aspects of dual phase continuity, have been the object of considerable scientific interest during the past decade. Earlier research at Lehigh University focused on I P N morphology via transmission electron microscopic ( T E M ) techniques. T h e experimental measurements were in good agreement with predictive equations derived to explain the dependence of the phase domain dimensions of polymer II on the crosslinking density of both polymers (4-8). T h e availability of small-angle scattering techniques such as small-angle light scattering (SALS), small angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) has yielded significant progress in the understanding of the molecular structure and morphological features of phase-separated polymer blends (9-14). A study of the metamorphosis of the IPN morphology of poly(crossbutadiene)-inter-poly(cross-styrene) (PB-PS) IPNs by SANS and T E M (5) showed that the SANS data exhibited a maximum in scattering intensity at the value of the scattering vector, K, equal to 0.01. This value corresponds to a scattering dimension of the order of 700-900 Â . Because the maximum was relatively constant as a function of polystyrene conversion, a model of interconnecting cylinders was evolved. This maximum disappears late in the conversion; thus the IPN has an increasingly irregular structure. The present work is part of a larger study to measure the phase domain dimensions of IPNs. F o r this chapter, poly (cross-butadiene)-inter-poly(cross-styrene)1 IPNs, polybutadiene-infer-poly(cross-styrene) semi-II 1 Formerly, this compound name was written polybutadiene-polystyrene. Nomenclature is discussed in Chapter 2 of this volume.


10.

FERNANDEZ ET AL.

155 IPNs Based on PB-PS

IPNs, and a polybutadiene-polystyrene chemical blend 1 were prepared in which the second polymer synthesized, polystyrene, was fully deuterated to produce contrast for SANS and permit the dimensions of the individual polymer domains to be evaluated. Electron microscopy of osmium tetroxide stained samples yields domain sizes and shapes. T h e two methods have different systematic errors and measure somewhat different aspects of the morphology; thus, comparisons are useful. The principles of neutron scattering as applied to the solution of poly mer problems were described previously (9, 15-17). The coherent intensity in a SANS experiment is described by the dif


ferential scattering cross section άΣΙάΐί

per unit solid angle per unit volume

of the sample in units of reciprocal centimeters. The quantity άΣΙάΩ

ex

presses the neutron scattering power of a sample and is the counterpart of the Rayleigh ratio R (Θ) used in light scattering. For polymer blends consist ing of deuterated polymer molecules dispersed in a protonated polymer matrix, SANS in the Guinier region arises from the contrast between the deuterated and the protonated species. A n expression was derived for SANS from labeled two-phase polymer blends (18,19). Thereby, the SANS technique can be used to measure both the conformation of the individual labeled chains within a domain and the dimensions of the individual phase domains. For an incompressible twophase polymer blend in which one of the phases is composed of a mixture of labeled and unlabeled molecules, the total scattering cross section is (14, 18, 19)

— ( Κ , φ0)

= φ0(1

- φΌ)(αη

+

- aD)2

NFSS(K)

asVv ΟΗ(1

-

ΦΌ)

+

ST(K)

ΟΌΦΌ

V

(1)

The system contains two polymer species: P, in which a fraction, φΌ, of molecules has been labeled (deuterated), and S, which consists of totally protonated molecules. T h e quantity asis the coherent neutron scattering length of monomer repeat units of the S species; a H is the coherent neutron scattering length of the protonated monomer repeat units of the Ρ species; aD is the coherent neutron scattering length of the deuterated monomer repeat units of the Ρ species; NP is Avogadro's number; S s is the single-chain 1 The term "chemical blend" here means one polymer polymerized in the presence of an other. Earlier literature denotes these as "graft copolymers," regardless of the actual extent of grafting. The corresponding terms "mechanical blend" and "solution blend" refer to mechani cally mixed and solution mixed blends, respectively.


156

MULTICOMPONENT POLYMER MATERIALS


form factor; ST is the structure form factor; V is the total volume of sam ple; and Vf and V s are the monomer repeat unit molar volumes of the Ρ and S species, respectively. T h e quantity Κ is equal to 4 7 τ λ - 1 sin (0/2), where λ is the neutron wavelength and θ is the angle of scatter. Equation 1 yields the phase-separated polymer domain scattering. The quantity ST(K) is proportional to the total scattering from a blend in which the Ρ species is fully deuterated ( φ ρ = 1). The materials studied were IPNs in which the first polymer synthe sized was a protonated polybutadiene [PB(H e )] network, S species, and the second polymer synthesized was deuterated polystyrene [PS(D 8 )]. For a sample in which all the Ρ species molecules have been deuterated

(*D - 1),

—(Κ,

φΌ -

1) - [αΌ

- as—)

(2)

If the monomer structural units (mers) of polybutadiene and polystyrene are - C 4 H 6 - and - C g D g - , respectively, the values of the mer scattering lengths are aD = 10.656 Χ 1 0 " 1 2 cm and as = 0.416 X 1 0 " 1 2 cm. This cross-section is proportional to the scattered intensity, which depends on the fluctuations in scattering length density in the solid and on the sizes of the regions over which these fluctuations occur. In such a case, the scatter ing can be described by the theory first introduced by Debye and co-work ers as a treatment of the scattering of radiation by an inhomogeneous solid material (20,21). T h e inhomogeneities present in a random two-phase ma terial may be characterized by a spatial two-point correlation function, which measures the degree of correlation between two fluctuations as a function of their distance of separation. T h e correlation function y(r) is given by the equation

y(r) ?2AV =

Interpenetrating Polymer Networks Based on Polybutadiene and

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