Macromolecules 1989,22, 3153-3161
3153
A Polydiacetylene in Dilute Solution Renliang Xut and Benjamin Chu*iti* Departments of Chemistry and Materials Science and Engineering, State University of New York at Stony Brook, Long Island, New York 11794-3400. Received November 7, 1988; Revised Manuscript Received December 21, 1988
ABSTRACT: Laser light scattering (LLS), transient electric birefringence (TEB), and optical absorption were used to study polydiacetylene P4BCMU (poly[ 1,2-bis[4-[[ [ (2-butoxy-2-oxoethyl)amino]carbonyl]oxy]butyl]-l-buten-3-yne-1,4-diyl]) in dilute chloroform/toluene solutions with various solvent compositions. Two different molecular weight P4BCMU samples were investigated. P4BCMU molecules in chloroform are wormlike coils with an average persistence length of -16 nm. When the mole fraction of chloroform in a chloroform/toluene mixture is smaller than -0.4, P4BCMU molecules in dilute solutions form aggregates, with molecular weight and size varying according to the solvent composition. Profile analysis of autocorrelation functions yields a broad size distribution when P4BCMU is dissolved in chloroform. However, much narrower size distributions are found once the aggregates are formed. Various models are applied to fit the experimental results, yielding mostly fairly stiff rodlike structures for the aggregates. The aggregation mechanism is discussed.
I. Introduction Polydiacetylenes, (=CR-C=C-CR'=),, polydiacetylene and its derivatives, are prototype conducting polymers' and can be used as a new class of electronic materials. They have been available since the late 1960s with the solid-state 1,4-addition polymerization of substituted diacetylenes.2 Most polydiacetylenesare insoluble even in exotic solvents and infusible because of their rotation-restricted stiff b a ~ k b o n e .After ~ the first soluble polydiacetylene, PnBCMU, in which R' = R = (CH2),0CONHCH2COOC4H9,was synthesized a decade there have been extensive studies on the electronic, optical, mechanical, and conformational properties of POlydiacetylenes in both solid (crystals and thin films) and liquid (gels and solutions) states. This research topic has also been the subject of several Applications of polydiacetylenes in materials science have become significant because of their many interesting properties, such as the large quasi-one-dimensional structure of their single crystals, the high third-order optical susceptibility, the large optical nonlinearity, and the high photoconductivity."" The color change of polydiacetylenes associated with either the solid-state polymerization by y-radiation or the conformational change can be used to monitor any one of the following ambient parameters: time-temperature exposure, humidity, pressure, radiation exposure, pH, and gas exposure. Commercial products that try to take advantage of the color change are time-temperature and radiation dosage indicators consisting of a diacetylene ink printed as a bar-code label of the type commonly used for automatic product identification and pricing in merchandising, but providing dynamic instead of static information.12-13 Polydiacetylenes have been used as a film waveguide in interferometer~.'~-'~ The potential use of polydiacetylenes as an optical memory and information processor has also been explored.16 The unique property to which most attention has been paid is the peculiar color change of polydiacetylenes in solution a t different temperatures or at different solvent conditions; e.g. solutions of P4BCMU in toluene at room temperature are red and will gradually turn to yellow by adding chloroform or by raising the temperature. Over 20 major techniques, e.g. nuclear magnetic resonance, Fourier *Author to whom all correspondence should be addressed (use De artment of Chemistry). PDepartment of Chemistry. Department of Materials Science and Engineering.
*
transform infrared spectroscopy, viscoelastic measurements, light scattering, transient electric birefringence, optical Kerr effect, and Raman and absorption spectroscopy, which cover a broad detection region from picosecond to millisecond in time and from angstrom to micrometer in space, have been applied to study the dramatic color change of more than 10 members, e.g. PnBCMU and PTS12 (R = (CH2),0S02BzCH3),of the soluble polydiacetylene family.''-27 Customarily known as P4BCMU ((C26H$20&, Mmono = 508 g/mol, L ,,, = 4.8 A26,poly[5,7-dodecadiyne-1,12bis[ [ (4-butoxycarbonyl)methyl]urethane]], or systematically named poly[ 1,2-bis[4-[ [ [ (2-butoxy-2-oxoethy1)amino]carbonyl]oxy]butyl]- 1-buten-3-yne-1,4-diyl], was first synthesized in 1978.4 It has bulky, sufficiently flexible and polar substituent groups R = (CH2)40CONHCH2COOC4HQ.The backbone configuration of P4BCMU is believed to be a resonance mixture of two mesomeric structures, i.e. the favored acetylenic structure and the much less favored butatrienic structure (ref 16 and references therein): (=RC--CrC--CR=),
++
(-RC=C=C=CR-),
It should be noted that the existence of the butatrienic form is being disputed. The crystal of P4BCMU has a quasi-one-dimensional structure because of the conjugated backbone. P4BCMU is highly soluble in chloroform (>5%) and forms a yellow solution with a blue shift in the optical absorption peak from ,A, = 625 nm of the original crystalline state28to ',mA = 460-470 nm. Dramatic reversible color changefrom yellow to red with an additional absorption a t Xmm2 = 520-550 nm-occurs by adding n-hexane (a nonsolvent) to P4BCMU/CHC13 ~olution.'~Subsequent addition of n-hexane results in the precipitation of the polymer as a red solid. The precipitation process varies considerably with polymer concentration, while the conformational transition (the color change) occurs at a fixed CHC13/C6H14 ratio over a range of concentrations spanning 3 orders of to 1 X low5g/mL. A similar magnitude from 3 x phenomenon is observed when P4BCMU is dissolved in toluene (a poor solvent). At high temperatures, the solution is yellow, and a t lower temperatures it becomes red. It has been found in many experiments that the size of P4BCMU molecules depends on solution color. Several theoretical models and calculations have been proposed to explain the p h e n o m e n ~ n . ~ "The ~ ~ focal points are the structures and transition mechanisms of polydiacetylenes in different environments. The conformation of soluble
0024-929718912222-3153$01.50/0 0 1989 American Chemical Society
3154 X u and Chu
polydiacetylenes in good solvent is clear; i.e., they are wormlike coils of small overall size (a few hundred angstroms) with a fairly broad size distribution. But how are t h e coils being transformed t o another state a n d what is the other state when the solvent condition becomes poorer? What is t h e relation between the electronic configuration, which displays the color, and the geometric conformation, which determines t h e particle size a n d shape? We shall a t t e m p t t o answer those questions in this article. Transient electric birefringence (TEB), as an important branch of modern electrooptics, with single, reversed or sinusoidal electric pulses has become a useful tool for studying t h e structure a n d t h e electrical, optical, a n d hydrodynamic properties a n d particle sizing of large anisotropic particles in solution (or suspension). TEB can determine t h e rotational diffusion coefficient a n d obtain information a b o u t t h e optical anisotropy of particles. From fluctuations of t h e dielectric constant, density or concentration of t h e scattering medium, laser light scattering (LLS) detects t h e scattered electric field, which results from t h e interaction of molecules with t h e incident light, as functions of time and space t o obtain information on t h e structures, e.g. t h e z-averaged radius of gyration (Q, t h e weight-averaged molecular weight (M,), a n d t h e second virial coefficient (A2),a n d on t h e translational as well as internal motions, e.g. t h e z-averaged translational diffusion coefficient (DT)a n d t h e rotational diffusion coefficient (DR),of t h e scattering particles in solution (or suspension). In t h e present study LLS is used t o study P4BCMU in mixed solvents of chloroform/toluene as functions of time and solvent composition. Optical absorption monitors t h e time a n d composition dependence of t h e solvatochromic transition. TEB is also employed to determine the amplitudes of t h e solution birefringence a n d particle rotational diffusion coefficient. T h e molecular status a n d possible transition mechanisms are t h e n drawn from t h e results of data analysis. Our studies on P 4 B C M U in chloroform/toluene dilute solutions as functions of time a n d composition show that coils of P4BCMU in relatively good solvents could become rodlike aggregates when t h e solvent condition becomes poorer. T h e transition happens when t h e mole fraction of chloroform in t h e solvent mixture is less t h a n -0.4; it takes a short time period ( 4.556
with x = L / p , and L, b, p, and a'being the contour length, the distance between adjacent frictional elements, the persistence length, and the Stokes diameter of an element, respectively. In eq 19 and 20, Al - A5 and C1 - Clo are all functions of (a'/p).= We have four floating parameters, x, p, b, and a', in three equations. Again, if the particle is a (partially) straightened molecule, then a' would be close to twice the side-chain length (-40 A) and b could be reasonably chosen as the length of the longer r-conjugation, which is about 53 A. But that is not the case. If we prefix the b value being 53 A, the sets of x, p, and a' obtained have much thicker diameters of -150 nm as shown in Table IV. This result again rules out the single-chain phenomenon in poorer solvents. During the iterative fittings it was found that for all the data sets there was only a limited range available for prefixed b values, which could be used while still obtaining stable and meaningful solutions (sets of L, p, and a?. This range is from -40 to -110 A. For a particular set of DR, DT, and R,, a' is quite stable, but increasing L (up to -15% bigger) and decreasing p (down to -40% smaller) were found when b was increased from 40 to 110 A. Table IV lists the values using a prefixed b of 80 A. The results are surprisingly close to those from the rod model. Although we started with a wormlike chain model, P4BCMU in these solutions was found to be fairly rigid, with a typical L / p value being only 2-4. The scattered intensity ratio for solutions with X, € 1 when compared with chloroform solutions (X, = l), as the values listed in the last column of Table 11, originates from both the change in the refractive index increment (en/&') of the solution and/or the aggregation of polymer molecules. Thus, the an/aC values are crucial in the determination of the aggregation number. Muller et al.25have extensively measured d n / X values for P4BCMU in toluene at various concentrations and wavelengths. They concluded that the value for P4BCMU in toluene at ho = 633 nm was only about twice the value for P4BCMU in chloroform. Due to the different refractive indices of toluene and chloroform and their different boiling points, measurements of refractive index increment for solutions in mixed solvents could not provide us with reliable values. So, we emphasize only the toluene solution. If we use the value of (dn/13C)x~=~ = 0.242 and assume that it is independent of the molecular weight of PIBCMU, then the average aggregation numbers are readily estimated for P4BCMU in toluene; nA= 360 and nB = 14. In the same
solvent condition (e.g. X,= O), the aggregation number of sample A is much higher than sample B. As reported and suggested before (ref 35 and the references therein), the neutral coil of polydiacetylene in a poor solvent (or solvent mixture) would be partially straightened to have longer n-conjugation; meanwhile, the molecules could become charge carriers in a solvent of a proper dielectric constant. The charge density (p,) is reciprocally proportional to the polymerization number, and charges are mainly located at the ends of the molecules (ref 35 and the references therein). Under electrostatic and solvation forces, those charged molecules would aggregate to form rodlike particles. According to this explanation, sample A would have a higher charge density than sample B and be easier to adhere with each other. Also, the structure of aggregates from sample A should be denser. Thus in a stationary (equilibrium) state, sample A is expected to have a higher aggregation number and denser structure. This explains the result of the aggregates from sample A having a large aggregation number (nA= 360 vs nB = 14 corresponding to MwA = 4.3 X lo7 g/mol vs Mw,B= 3.4 X lo7 g/mol) but at smaller dimensions ( ( R J A = 229 nm vs (RJB= 268 nm) than those from sample B. When the above aggregation numbers for sample A and sample B are combined with the aggregation number reported by h w k o et al.,22a linear empirical relation between the average polymerization number P, and the aggregations number n in toluene, ( n - 1) = 8.5 X lO4PW-' a pe (charge density), could be extracted. The reasons for the different molecular weight samples approaching similar size and shape are still not clear. The narrower size distribution for the rod aggregates does suggest a closed association of micelle formation for P4BCMU in poor solvent.
V. Conclusion and Remarks By LLS and TEB measurements and model fittings of measured data, we have experimentally confirmed that P4BCMU polymer molecules in good and poor solvent exist in single wormlike chains and uniform rodlike aggregates, respectively. The concentration dependence of particle size in dilute solution is very weak. This finding agrees with previous results.21,22The aggregation number is molecular weight and solvent composition dependent. The transformation (color and size change) starts at X, 5 0.43. The size of aggregates varies with solvent composition. The monodisperse behavior of aggregates suggests that the aggregation process might be micellizationlike. Further investigation of the transition of polydiacetylene in solution over a much shorter time range is needed to clarify the transition mechanism. Acknowledgment. We thank Drs. R. R. Chance and D. G . Peiffer who provided us the polydiacetylene samples. This work was supported by the Polymers Program of the National Science Foundation (DMR8617820). Appendix
Conversion of DT Distribution to M Distribution. By using the relation DT = k f l - * D with k D and CYDbeing two scaling constants, we can convert the translational diffusion coefficient (DT) distribution to the molecular weight (M) distribution as follows. The discrete normalized linewidth distribution from photon correlation is
where Ni, Mi, and ri(=P?DT,i,K is the magnitude of the scattering vector) are the numbers of molecules, the mo-
Polydiacetylene in Dilute Solution 3161
Macromolecules, Vol. 22, No. 7, 1989 lecular weight, and the linewidth, respectively, of the ith species. Substituting eq A1 into the definition of the weight-averaged molecular weight (M,), we get
M, =
C F ( M i ) =-= CNiMi2 C F ( M i )/ M i CNiMi
McWhirter and Pike5' have showed that the behavior of a logarithmically spaced discrete distribution F ( r i )could be equalized to a continuous distribution G ( r )after each point of F ( r i )is corrected by multiplying a factor i.e. F ( r i ) = riG(ri). Thus, in a logarithmically spaced continuous distribution from the CONTIN algorithm, eq A2 could be rearranged to yield
The left side of eq A3 could be determined by using static LLS. can then be obtained from a ratio of two samples of different molecular weight
With cyD, k D can be determined from eq A3. Following a similar route, M,, can be calculated by
M , = - -CNiMi ZNi
-
CF(ri)/Mi CF(I'i)/M:
The continuous MWD, G ( M ) ,is related to G ( r ) according to the relation G(M) = I'G(I')/M (A6)
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