*Corresponding author. Tel.: #81-298-64-5614; fax: #81-298-64-3202.

E-mail address: naoya.torikai@kek.jp (N. Torikai)

Physica B 283 (2000) 12}16

Interfacial structures of block and graft copolymerswith lamellar microphase-separated structures

Naoya Torikai!,*, Yushu Matsushita", Sean Langridge#, David Bucknall#,Je! Penfold#, Masayasu Takeda$

!Neutron Science Laboratory, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan"School of Engineering, Nagoya University, Nagoya, 464-8603, Japan

#ISIS Facility, RAL, Chilton, Didcot, Oxon, OX11 0QX, UK$Physics Department, Tohoku University, Sendai, 980-8578, Japan

Abstract

Chain conformations of A-polymers of BAB two-component triblock and ABB graft copolymers, composed ofpolystyrene ("A) and poly(2-vinylpyridine) ("B), with lamellar microphase-separated structures were investigated bysmall-angle neutron scattering (SANS), and interfacial thicknesses of AB diblock and BAB triblock copolymers werecompared by using neutron re#ectivity (NR). The middle A-block of BAB triblock copolymer has preferentially `loopaconformation when the volume fraction, /

A, of A-chain is high above 0.5, while it does preferentially `bridgea

conformation when /A

is low around 0.3. A-graft chain of ABB copolymer has slightly larger dimension along thedirection parallel to lamellar interface than A-chain of AB diblock copolymer. The interfacial thickness of BAB triblockcopolymer is almost constant irrespective of /

A, and comparable with that of AB diblock copolymer. ( 2000 Elsevier

Science B.V. All rights reserved.

Keywords: Block and graft copolymer; Microphase-separated structure; Chain conformation; Interfacial thickness; Small-angle neutronscattering; Neutron re#ectivity

1. Introduction

It has been well-established that block and graftcopolymers composed of immiscible polymers formmicrophase-separated structures with various morpholo-gies, e.g., spherical, cylindrical, lamellar structures, etc.,according to their composition in bulk. The microphase-separated structure has been studied and understood atthe molecular level intensively on AB diblock copolymerso far [1}4]. For example, it has been clari"ed by small-angle neutron scattering (SANS) that block chain inlamellar microdomain is stretched along the direction

perpendicular to lamellar interface, and is shrunk alongthe direction parallel to the interface [1].

BAB two-component triblock copolymer has twochemical junction points to be restrained at domaininterface, so that its middle A-block chain must takeeither `bridgea conformation or `loopa one, in which thetwo junction points are anchored on the adjacent domaininterfaces or on the same one, respectively. On the otherhand, ABB graft copolymer has a peculiar structuralfeature that one A- and two B-chains are connected atone junction point, so that the chain distribution atdomain interface is asymmetric. In our previous works[5,6], it was clari"ed that composition dependences ofmorphology for BAB and ABB copolymers di!er fromthat for AB diblock copolymer even if they are composedof the same pair of polymers. Thus, it is signi"cant fora further understanding of microphase-separated struc-ture to investigate chain conformations and interfacial

0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 8 8 2 - 7

Fig. 1. Geometrical relationship among an irradiated neutronbeam, "lm specimens, and a two-dimensional position sensitivedetector in SANS measurement. t is the azimuthal angle on thedetector.

Fig. 2. (a) SANS through-view pro"le for DPP/SPP blend at theweight fraction of 0.13/0.87. (b) Its Guinier plots. The brokenvertical line indicates the upper limit of the Guinier rangede"ned as q2R2

',,(1.32/3.

structures of BAB and ABB copolymers, and comparethem with those of AB copolymer.

In this study, we investigated chain conformations ofA-chains of BAB triblock and ABB graft copolymers,composed of polystyrene ("A) and poly(2-vinylpyridine)("B), in microdomain by SANS, and also comparedinterfacial thicknesses of AB diblock and BAB triblockcopolymers by neutron re#ectivity (NR).

2. Experimental details

The samples used are DP diblock, PDP and PSPtwo-component triblock, and DPP and SPP graftcopolymers, in which D, S, and P denote poly(styrene-d8), poly(styrene-h

8), and poly(2-vinylpyridine), respec-

tively. All the samples were prepared by an anionic pol-ymerization method.

For SANS, several mixtures of PDPs and PSPs wereprepared at the weight fraction of PDP around 0.11 to"nd out the best contrast-matched composition. As forgraft copolymer, only one DPP/SPP mixture, whoseweight ratio is 0.13/0.87, was prepared. Film specimens(0.2 mm thick) of these blends were prepared by solvent-casting from THF solutions, and then were annealed at1503C in vacuum for one week. It was con"rmed bysmall-angle X-ray scattering that lamellar structures arepreferentially oriented along the direction parallel to "lmsurface. SANS measurement was performed on SANS-Uspectrometer of the Institute for Solid State Physics, theUniversity of Tokyo, using a two-dimensional positionsensitive detector. The measurement was performed atthe through and edge views, in which the neutrons areirradiated along the direction perpendicular and parallelto "lm surface, respectively, as schematically shown inFig. 1.

Thin "lms (&1200 As thick) of DP and PDPs for NRwere prepared by spin-casting on cleaned surfaces ofsilicon wafers. These "lms were fully annealed at a tem-perature above ¹

'before measurement. NR measure-

ment was performed at RT using two re#ectometersinstalled at pulsed- neutron source: CRISP of RutherfordAppleton Laboratory and TOP of High Energy Acceler-ator Research Organization.

3. Results and discussion

Two-dimensional SANS patterns at the through viewfor PDP/PSPs and DPP/SPP were all isotropic, so thatthe scattering intensity was circularly averaged. Fig. 2(a)shows the through-view pro"le for DPP/SPP, where qis the scattering vector de"ned as 4p sin h/j, and 2h isthe scattering angle. As noted from the "gure, di!ract-ion peaks from lamellar structure are not observed inthe pro"le. Similarly, the through-view pro"les forPDP/PSPs have no di!raction peaks, though the pro"les

N. Torikai et al. / Physica B 283 (2000) 12}16 13

Table 1The radii of gyration of middle styrene block chains of PDP/PSP triblock copolymers, and a styrene graft chain of DPP/SPP copolymer

Sample! M"-0#,

" (]10~3) / R',9

(As ) R',:

(As ) R',:

/R',9

R',,0

# (As )

PDP/PSP 31.0 0.27 20.6 51.0 2.46 29.130.6 0.32 24.8 41.1 1.66 28.944.3 0.50 32.5 38.5 1.23 34.773.5 0.56 38.3 54.1 1.44 44.7

DPP/SPP 169 0.54 54.2 * * 67.8

!The molecular weight distributions, M8/M

/, of all the triblock and graft copolymers used are less than 1.05.

"M"-0#,

is the molecular weight of styrene chain reduced to poly(styrene-h8).

#R',,0

is the radius of gyration of an unperturbed styrene chain along the k-axis. R',,0

"0.165M1@2 (As ).

Fig. 3. Specular re#ectivity pro"le for DP diblock copolymer.The solid line is the best-calculated re#ectivity from the b/vpro"le shown in the inset. In the inset the origin of abscissacorresponds to air surface of "lm.

are not shown here. On the other hand, edge-viewpatterns for PDP/PSPs were apparently anisotropic, sothat scattering intensity was sector-averaged aroundazimuthal angle, t, on the detector. The best contrast-matched blends showing no di!raction peaks were se-lected for respective PDP/PSPs by comparing the edge-view pro"les at t"03$53 and 1803$53 to estimateR

'component perpendicular to the lamellar interface.

The SANS pro"le without di!raction peaks was as-sumed to consist of only single-chain scattering frompolystyrene. Thus, the radius of gyration, R

',,, of styrene

chain along the k-axis was evaluated from the scatteringintensity according to the modi"ed Guinier's approxima-tion

I(q)"I(0) exp(!q2R2',,

), (1)

where k"x, y, or z in the coordinates shown in Fig. 1. Asan example, the Guinier plots of the scattering intensityin Fig. 2(a) are shown in Fig. 2(b). The radii of gyration,R

',9and R

',:, of styrene chain along the direction parallel

and perpendicular to lamellar interface were evaluatedfrom the circularly averaged through-view data andsector-averaged edge-view data at t"03$53 and1803$53, respectively. The results are summarized inTable 1. It was found that the R

',:/R

',9ratio is increasing

with decreasing the volume fraction, /, of styrene block,and the volume (JR2

',9R

',:) occupied by the segments of

the middle block chain is equal to that (JR3',,0

) of theunperturbed chain with the same molecular weight irre-spective of /. Though it is impossible to distinguishbetween `loopa and `bridgea conformations by the pres-ent SANS technique, we can speculate that the middleblock chain of triblock copolymer has preferentially`loopa conformation when / is high, while it has prefer-entially `bridgea conformation when / is low, since it canbe easily considered that apparent R

',:of the chain with

`bridgea conformation could be larger than that of thechain with `loopa one. On the other hand, the apparentR

',9of a styrene graft chain was evaluated to be 54.2 As .

This value is slightly larger than that, 51.2 As , for styrene

block with the same molecular weight, 169]103, of SPdiblock copolymer calculated from the empirical rela-tionship, i.e., R

',9"0.289M0.43 As . Thus, A-graft chain of

ABB copolymer in microdomain may be less deformedfor compensating asymmetric chain distribution at theinterface owing to its molecular architecture.

Fig. 3 shows specular re#ectivity pro"le for DP dib-lock copolymer as a function of neutron momentumtransfer, q

z("4p sin h/j), along the direction perpen-

dicular to the "lm surface, where h is the incident angle ofneutrons to "lm surface. The experimental pro"le, repre-sented by the open circles, exhibits several Bragg peakssuperimposed on the fringes with high-frequency re#ect-ing total "lm thickness. The existence of distinct Braggpeaks of relatively higher order implies that lamellarstructures are preferentially oriented along the directionparallel to the "lm surface. The solid line in the "gure isthe best re#ectivity calculated from the coherent scatter-ing length density, b/v, pro"le shown in the inset. TheD-lamella with b/v"6.1]10~6 As ~2 and P-lamella withb/v"2.0]10~6 As ~2 are alternatively stacked with the

14 N. Torikai et al. / Physica B 283 (2000) 12}16

Fig. 4. Specular re#ectivity pro"les for PDP triblockcopolymers with di!erent compositions. The broken and solidlines are the best-calculated re#ectivities for PDPs with /"0.50and 0.59, respectively. The inset shows the corresponding b/vpro"les used for the calculation.

structural repeating period of 430 As through a 1100 As -thick "lm. There could be no islands or holes on thesurface of this DP "lm, since the quality of "t becomesworse when the islands or holes are considered in thecalculation of re#ectivity. The interfacial thickness, t

I,

de"ned as

tI"1/(d/

i(z)/dz)

(i/0.5, (2)

where /i(z) is the variation of volume fraction of i-

component at interface, was evaluated to be 33 As . Fig.4 compares specular re#ectivity pro"les for three PDPswith di!erent compositions, i.e., /"0.34, 0.50, and 0.59.It is noted that the Bragg peaks for PDP with /"0.34are weaker than those for the other two PDPs implyingthat the lamellar structure is less oriented along thedirection parallel to the "lm surface or less ordered. Inthe actual data analysis for PDP with /"0.34, the good"tting was not obtained when it was assumed that D- andP-lamellae in the "lm have the b/v values for their puresegments and the same structural units are repeatedthrough the "lm. Thus, interfacial thickness was evalu-

ated from the other two PDPs. The broken and solidlines in the "gure are the best-calculated re#ectivities forPDPs with /"0.50 and 0.59, respectively, and the insetshows the corresponding b/v pro"les used for the calcu-lation. It was found that relative intensities of the Braggpeaks are determined by /, i.e., ¸

D/¸, where ¸

Dand ¸ are

the thickness of D-lamella and domain spacing, respec-tively, and the values of /(,¸

D/¸) evaluated from NR

results are consistent with the analytical values. Theevaluated t

Ivalues for the two PDPs are 30 and 32 As ,

and they are in good agreement with the result of de Jeuet al. [7] on PDP with M"120]103 and /"0.48. Theinterfacial thickness for PDP is almost constant irre-spective of /, and comparable with that for DP implyingthat the di!erence in the conformation of the middleblock chain does not have much in#uence on the inter-facial structure.

In the theory of Helfand}Wasserman [8], blockcopolymer interface in the strong segregation limitis approximated to be the same as the interfacebetween homopolymers having in"nite molecularweights. In their theory, /

i(z) is predicted to be a hyper-

bolic tangent function, and the interfacial thickness,tI,HW

, is given by

tI,HW

"2a/(6s)1@2, (3)

where a is the Kuhn statistical length and s is theFlory}Huggins interaction parameter. The t

I,HWis evalu-

ated to be 17 As from Eq. (3) by introducing 6.8 As and 0.11(at 298 K) for a and s, respectively. This value is appar-ently smaller than the experimental values for DP andPDPs. Thus, t

I,HWwas corrected by considering connect-

ivity of block chain and interfacial #uctuation accordingto the approach of Shull et al. [9]. The interfacial thick-ness, t

I,#0, corrected for connectivity was evaluated from

Fig. 4 of Ref. [10] directly, where the tI,#0

/tI,HW

ratiopredicted by the mean-"eld theory is plotted as a func-tion of sN, where N is the degree of polymerization ofblock copolymer. The magnitude, *t

I,&-, of interfacial

#uctuation is given by

(*tI,&-

)2"(kB¹/2pc) ln(j

.!9/j

.*/), (4)

where kB

is the Boltzmann constant, ¹ is the absolutetemperature, c is the interfacial tension, and j

.!9and

j.*/

are the upper and lower limits of wavelengths of#uctuations, respectively. In the calculation of *t

I,&-,

c was estimated according to the equation, i.e.,c"aok

B¹(s/6)1@2, where o is the number density of poly-

mer, and lamellar domain spacing and tI,HW

were used asj.!9

and j.*/

, respectively. The results are summarized inTable 2, along with the experimental data. It was foundthat t

I,#0`&-values thus evaluated are very close to the

experimental tI,%9

values irrespective of the moleculararchitecture of copolymers.

N. Torikai et al. / Physica B 283 (2000) 12}16 15

Table 2Comparison between experimental and theoretical interfacial thicknesses of DP diblock and PDP triblock copolymers

Sample M! (]10~3) / s" sN tI,%9

(As ) tI,HW

(As ) tI,#0

(As ) tI,#0`&-

(As )

DP 88.0 0.50 0.11 89 33$3 17 21 31PDP 88.5 0.50 0.11 90 30$2 17 21 30

94.4 0.59 0.11 95 32$2 17 20 30

!M is the total molecular weight of block copolymer."The s value (at 298 K) in the table was estimated from it's ¹-dependence determined by SANS.

Acknowledgements

N.T. is greatly indebted to Prof. M. Imai ofOchanomizu University and Mr. M. Nagao of ISSP, TheUniversity of Tokyo, for their technical support in SANSexperiment.

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