Manipulating the Thermoresponsive Behavior of Poly(N

Chapter 13

Downloaded by NORTH CAROLINA STATE UNIV on September 24, 2012 | http://pubs.acs.org Publication Date: November 28, 2000 | doi: 10.1021/bk-2001-0780.ch013

Manipulating the Thermoresponsive Behavior of Poly(N-isopropylacrylamide) *

C.K. Chee, S. Rimmer, I. Soutar and L. Swanson , The Polymer Center, School of Physics and Chemistry, Lancaster University, Lancaster LA1 4YA UK

Fluorescence techniques, including quenching and time -resolved anisotropy measurements have been used to follow the temperature induced conformational transition from an open coil to globular structure, of poly(N -isopropylacrylamide), PNIPAM. The onset of the coil collapse occurs at 32°C, the lower critical solution temperature (LCST). In view of the potential of such polymers to act as carriers in controlled release applications, it would be attractive if ways could be found to manipulate the LCST of the polymeric host through chemical modification. In this paper, we present the results of initial attempts to achieve this aim by copolymerization of NIPAM with varying amounts of styrene. Fluorescence spectroscopy has shown that alteration of the hydrophobic/hydrophilic balance of NIPAM-based polymers, through random copolymerization, changes the LCST of the thermoresponsive polymer. Unfortunately, the magnitude of the transition, is also reduced in such cases. Manipulation of the polymer topography, on the other hand, seems to offer an alternative route whereby the LCST of smart systems may be controlled.

© 2001 American Chemical Society

In Stimuli-Responsive Water Soluble and Amphiphilic Polymers; McCormick, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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Introduction Polymers which exhibit "smart" behavior (i.e. respond to external stimuli such as pH and/or temperature) have attracted much interest in both the academic and industrial communities over a number of years (1,2). Much of this attention stems from the fact that these systems are fundamentally interesting, and also are industrially important. They are involved in a diverse range of technologies such as controlled-release systems, agrochemicals, adhesives, coatings, enhancers for oil recovery, foodstuffs, rheology modifiers, personal care products, superabsorbents, catalysis, inks and coding systems. The use of fluorescence spectroscopy (3,4) has been particularly prominent in investigation of smart polymers since it allows examination of ultra-dilute solutions, permitting examination of purely w/ra-molecular effects. Indeed, luminescence techniques have proved invaluable in confirming (5,6) that poly(methacrylic acid), PMAA, undergoes a pH-induced conformational transition from an uncoiled (at high pH) to a hypercoilca structure at pH 4. With poly(acrylic acid) (7) this transition is much less dramatic: the polymer coil essentially adopts an open chain conformation at all values of pH. Poly(N-isopropylacrylamide), PNIPAM, undergoes a similar conformational transition to that of PMAA, except that contraction and expansion of the coil is controlled by temperature. The onset of the coil collapse occurs at 32°C, the lower critical solution temperature (LCST) (8,9). Under semi-dilute conditions, the polymer forms a turbid solution upon heating above the LCST, which rapidly turns clear again upon cooling. In this respect, we have recently confirmed (10) (via time-resolved anisotropy measurements, TRAMS) that the LCST behavior in linear PNIPAM is governed by a 2-stage mechanism (11,12). The first step involves intramolecular coil collapse. This is followed by intermolecular aggregation between collapsed coils. The ability of "smart" polymers to expand and contract "on demand" could lead to such systems being used as carriers with controlled release capabilities. For example, in the compact form, above its LCST, PNIPAM can solubilize low molar mass organic species (13). The solute can subsequently be released into the aqueous phase by simply lowering the temperature of the dispersion below 32°C. In view of this potential, it would be attractive if ways could be found to manipulate the LCST of the polymeric host through chemical modification. We have recently modified PNIPAM (14,15) and synthesized microgels (16) based upon PNIPAM so that we can control the conformational switch of the polymer over a wide temperature range (e.g.from4-100 °C) including the physiological temperature of 37°C. The resultant modified polymers might then find application in a much more extended range of industrial and medical activities. In this paper, we present the results of initial attempts to achieve this aim.


225 Experimental


Materials N-isopropylacrylamide, NIP AM, (Aldrich; 97%) was purified by multiple recrystallization from a mixture (60/40 %) of toluene and hexane (both spectroscopic grade; Aldrich). Styrene was purified by washing with an aqueous solution of NaOH (5 wt%) to remove inhibitor, washed with distilled water until the washings were neutral to litmus andfractionallydistilled under high vacuum. Acenaphthylene (ACE) was purified by multiple recrystallization from ethanol followed by vacuum sublimation. Nitromethane (Aldrich; Gold Label) was used as received. Acenaphthylene labeled poly(N-isopropylacrylamide) [ACE-PNIPAM] was prepared by copolymerisation of NIPAM with a trace amount (ca. 0.5 mole %) of ACE in dioxane solution (80% by weight of solvent) at 60°C using AIBN as initiator. Fluorescently labeled styrene-NIPAM copolymers were prepared in a similar manner to that of the homopolymer. Two samples were synthesized: one contained 8.9 weight % styrene [ACE-(STY8.9)-NIPAM] while the second contained 16.9 weight % of hydrophobe [ACE-(STY16.9)-NIPAM]. All polymers were purified by multiple reprecipitation from dioxane into diethylether (May and Baker). Graft copolymers containing a styrene backbone and NIPAM side chains were prepared by a macromonomer (17) technique and purified by ultra filtration. Details will be published (14) in due course. Contents of all copolymer samples were obtained by proton NMR and elemental analyses. Molecular weights were determined by aqueous GPC (14). The data are listed in Table 1. Table 1. Physical characteristics of the NIPAM based polymers. Sample ACE-PNIPAM ACE-STY(8.9)-NIPAM ACE-STY(16.9)-NIPAM ACE-STY(16)-gNIPAM STY(14)-gNIPAM-ACE

M gmoV n

(xlOOO) 21 19 21 1800 1700

LCSTfC)

Styrene content (%) 8.9 16.9 16.0 14.0

32 20 9 37 37

Instrumentation Optical density measurements were made on a Hitachi U-2010 spectrophotometer.


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Steady state fluorescence spectra were measured on a Perkin-Elmer LS50 spectrometer. Fluorescence lifetime data were acquired on an Edinburgh Instruments 199 time-correlated single photon counter. Time-resolved anisotropy measurements (TRAMS) were made at the synchrotron radiation source, SRS, Daresbury, UK. A complete description of the experimental set-up and a detailed discussion of analysis of anisotropy data can be found elsewhere (18).

Results and Discussion

(i) Behavior in methanol

Fluorescence quenching The accessibility of a fluorescent species, F, to a low molar mass species, Q, (which is capable of dynamic quenching of an excited state) can be estimated from quenching experiments. This technique follows the general principle outlined below. The quenching process may be represented by equation (1). F* + Q — • F + Q*

(1)

The efficiency with which a dynamic quencher accesses the excited state is described by the Stern-Volmer equation: τ7τ=1+^τ°[

Manipulating the Thermoresponsive Behavior of Poly(N

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