How Can Multi-Detector SEC Inform our Knowledge of Mechanochemical Polymer Degradation?

Transcription

1 How Can Multi-Detector SEC Inform our Knowledge of Mechanochemical Polymer Degradation? André M. Striegel National Institute of Standards & Technology

2 Objectives Use size-exclusion chromatography with multiangle static light scattering and viscometric detection to study effects of polymer architecture on transient elongational flow degradation. Attempt to isolate effects of long-chain branching (LCB) and conformation on degradation. Use ultrasonication as representative transient elongational flow degradation technique. Combine multi-detector SEC and ultrasonication to examine consequences for M lim.

3 The Rayleigh-Plesset Equation RR 3 2 R 2 1 p g P 0 P( t) R 4 R 2 R R R R radius of bubble dr dt 2 d R dt 2 p g gas pressure inside bubble P0 ambient pressure P( t) time dependent external p ressure kinematic viscosity of liquid surface tension of liquid density of liquid

4 Ultrasonication Adapted from K. S. Suslick, in Encyclopedia of Physical Science and Technology, 3 rd ed., R. A. Meyers, ed. (2001)

5 Temporal Evolution of Bubble Radius and Strain Rate During Cavitational Bubble Collapse Adapted from TQ Nguyen, H-H Kausch, Int. J. Polym. Anal. Charact., 4 (1998) 447

6 Interest in Ultrasonic Degradation Bubble collapse gives rise to large shear gradients responsible for ultrasound-induced bond scission. Collapse of cavitation bubble creates transient elongational flow. Two general types of flow fields: Shear Extensional Two types of extensional flow Steady state Transient elongational (a.k.a. fast transient) Note that shear field polymer degradation is extremely rare.

7 Interest in Ultrasonic Degradation Collapse of cavitation bubble creates transient elongational flow. In transient elongational flow, dwell time of fluid element very short compared to relaxation time of macromolecule. Results in non-random, near-midchain scission of linear polymers. Laboratory-created flow fields strictly analogous to those found in many real world scenarios.

8 Advantages of Ultrasonication Fluid mechanics and kinetics: Comparable to other means of generating transient elongational flows. a Relatively simple to isolate individual experimental variables. Minimal sample consumption. a TQ Nguyen, QZ Liang, H-H Kausch. Polymer, 38 (1997) 3783 TQ Nguyen, H-H Kausch. Adv. Poly. Sci., 100 (1992) 73

9 ULTRASONIC DEGRADATION Contrary to all chemical or thermal decomposition reactions, ultrasonic depolymerization is a nonrandom process that produces fragments of definite molecular size. Cavitation provides the most effective source of mechanical energy capable of causing the specific degradation of macromolecules AM Basedow, KH Ebert, Ultrasonic degradation of polymers in solution, Adv. Polym. Sci., 22 (1977)

10 Our Lab: The Last 15 Years AM Striegel, J. Biochem. Biophys. Methods 56 (2003) AM Striegel, Biomacromolecules 8 (2007) SG Ostlund, AM Striegel, Polym. Degrad. Stab. 93 (2008) AM Striegel, J. Liq. Chromatogr. Rel. Technol. 31 (2008) MJ Morris, AM Striegel, Polym. Degrad. Stab. 97 (2012) AM Striegel, J. Chromatogr. A 1359 (2014)

11 Influence of Long-Chain Branching on Transient Elongational Flow Degradation

12 Ultrasonication of Branched Polymers Dextran (Lorimer et al. 1995, Côté & Willett 1999) Guar galactomannan (Tayal & Khan 2000) Xanthan (Sohn et al. 2001) Poly(vinyl acetate) (Madras et al. 2000, 2001 ) Cellulose derivatives (Schittenhelm & Kulicke 2000) Starch derivatives (Schittenhelm & Kulicke 2000) Other long-chain branched glucans (Zhan et al. 2001)

13 POLYSTYRENE: Linear vs. Star 65,000 g/mol linear vs. 78,000 g/mol 8-arm star (ea. arm: 9,800) 257,000 g/mol linear vs. 255,000 g/mol 3-arm star (ea. arm: 85,000) vs. 202,000 g/mol 8-arm star (ea. arm: 25,300) 447,000 g/mol linear vs. 364,000 g/mol 8-arm star (ea. arm: 45,500) SG Ostlund, AM Striegel. Polym. Degrad. Stab., 93 (2008) 1510 AM Striegel. J. Biochem. Biophys. Methods, 56 (2003) 117

14 SEC/MALS/VISC/DRI MALS Solvent Reservoir (DMAc/0.5% LiCl) Pump Injector Thermostatted SEC columns DRI VISC 47 khz, 185 W Fluid connection Electronic connection

15 Comparison 1: PS-L 65 vs. PS-St8 9.8/78 PS-L 65: Linear PS ~65,000 g/mol PS-St8 9.8/78: 8-arm star PS ~78,000 g/mol, each arm ~9,800 g/mol

16 Results Sonicated for 4 hrs (240 min) PS-L 65: ~2% decrease in molar mass Increase in M w /M n from to Decrease in [] w from 0.22 to 0.21 dl/g PS-St8 9.8/78: No changes in MMD, polydispersities, etc. Conclusion: Limiting molar mass 65,000 g/mol

17 Limiting Molar Mass, M lim Molar mass beyond which macromolecules show no or negligible transient elongational flowinduced (ultrasonic) degradation Known to depend on chemical identity of polymer Also depends on physicochemical factors: Ultrasonic intensity Analyte concentration Solvent parameters: Ionic strength, temperature, viscosity, heat of vaporization, etc. Very limited information on architectural effects

18 Limiting Molar Mass, M lim Definition #1 A given polymer will not degrade in transient elongational flow if M M lim or Given two polymers of the same chemistry, if M1 > M lim and M2 M lim, Polymer 1 will degrade, but Polymer 2 will not. For a given set of experimental conditions

19 Comparison 2: PS-L 257 vs. PS-St3 85/255 vs. PS-St8 25.3/202 PS-L 257: Linear PS ~257,000 g/mol PS-St3 85/255: 3-arm star PS ~255,000 g/mol, each arm ~85,000 g/mol PS-St8 25.3/202: 8-arm star PS ~202,000 g/mol, each arm ~25,300 g/mol

20 DRI Response (arbitrary units) DRI Response (arbitrary units) DRI Response (arbitrary units) 3 2 PS-L min 10 min 20 min 40 min 120 min 240 min 360 min [] w (dl/g) R (nm) M w x 10 5 (g/mol) PS-St3 85/255 0 min 5 min 10 min 20 min 40 min 120 min 240 min [] w (dl/g) R (nm) M w x 10 5 (g/mol) Retention volume (ml) Retention volume (ml) 4 3 PS-St8 25.3/202 0 min 10 min 20 min 40 min 120 min 240 min [] w (dl/g) R (nm) M w x 10 5 (g/mol) Retention volume (ml) AM Striegel, J. Biochem. Biophys. Methods 56 (2003)

21 Comparison 3: PS-L 447 vs. PS-St8 45.5/364 PS-L 447: Linear PS ~447,000 g/mol PS-St8 45.5/364: 8-arm star PS ~364,000 g/mol, each arm ~45,500 g/mol

22 DRI Response (arbitrary units) DRI Response (arbitrary units) PS-St8 45.5/364 0 min 5 min 10 min 20 min 40 min 120 min 240 min 360 min 600 min [] w (dl/g) R (nm) M w x 10 5 (g/mol) Retention volume (ml) PS-L min 5 min 10 min 20 min 40 min 120 min 240 min [] w (dl/g) R (nm) M w x 10 5 (g/mol) AM Striegel, J. Biochem. Biophys. Methods 56 (2003) Retention volume (ml)

23 Degradation of Linear vs. Star PS LCB appears to affect mechanism of degradation. Mechanism of degradation of high-m 8-arm star: Resembles degradation of intermediate-m 3-arm star more than it does degradation of intermediate-m 8-arm star. LCB effect on M lim.

24 Revising the Definition of M lim Need to include LCB effects on M lim Propose (initially) that M path > M lim for ultrasonic degradation to occur.

25 core arm M arm» M core

26 If M arm > M lim, then degradation will occur If M arm < M lim, but 2M arm > M lim, then degradation will occur But, if 2M arm < M lim, then no degradation occurs, regardless of M star M arm» M core

27 M arm < M lim M lim 65 Kg/mol M arm 25.3 Kg/mol

28 2M arm M lim M lim 65 Kg/mol 2M arm 50.6 Kg/mol

29 8M arm» M lim M lim 65 Kg/mol 8M arm 202 Kg/mol

30 DRI Response (arbitrary units) DRI Response (arbitrary units) 4 3 PS-St8 25.3/202 0 min 10 min 20 min 40 min 120 min 240 min Retention volume (ml) 3 2 PS-St3 85/255 0 min 5 min 10 min 20 min 40 min 120 min 240 min Retention volume (ml) M arm = 85.5 Kg/mol 2M arm = 171 Kg/mol» M lim AM Striegel, J. Biochem. Biophys. Methods 56 (2003)

31 Limiting Molar Mass, M lim Definition #2 A continuous path of a given length must exist within a polymer, such that, if M path > M lim, the polymer will degrade or Given two polymers of the same chemistry and molar mass, if M path 1 > M lim and M path 2 M lim, Polymer 1 will degrade, but Polymer 2 will not. For a given set of experimental conditions

32 Monitoring Polymer Architecture During Degradation using SEC/MALS/VISC/DRI

33 Polymer Conformation & Dimensionless Size R G Radius of Gyration 1 n 1 i i R cm 2 r i : location individual atom or group of atoms R cm : location of polymer center of mass n: degree of polymerization r More compact structure R R G Hard sphere 1.3 Random coil of homopolymers Viscometric Radius R M N A []: intrinsic viscosity M: molar mass More extended structure Rigid rod Striegel, Yau, Kirkland, Bly, Modern Size-Exclusion Liquid Chromatography, 2 nd ed. (Wiley, 2009).

34 Ultrasonic Degradation: Architectural Effects 0.9 PS-L 65 PS-L 257 PS-L 447 R R 0.8, w G, z PS-St3 85/255 PS-St8 25/202 PS-St8 46/ Sonication time (min) R R, w G, z Sonication time (min) AM Striegel, J. Biochem. Biophys. Methods 56 (2003)

35 Calculating Degradation Rate Constants 1 M n, t k' k M o 1 M n,0 k' t M n,t = Number-average molar mass (in g/mol) after t min of sonication M n,0 = Number-average molar mass (in g/mol) of unsonicated sample (i.e, at t = 0 min) t = Sonication time (in min) M o = Molar mass of repeat unit (for PS, 104 g/mol) k = Degradation rate constant (in min -1 ) Plot 1/M n,t vs. t to obtain k and, consequently, k SL Malhotra, Ultrasonic solution degradations of poly(alkyl methacrylates), J. Macromol. Sci.-Chem., A, 23 (1986)

36 Effect of Molar Mass and Architecture on Degradation Rate Sample k (x 10-6 min -1 ) PS-L ± 0.00 PS-St8 9.8/78 No degradation observed PS-L ± 0.04 PS-St3 85/255 k 1 = 0.87 ± 0.11 (0-20 min) k 2 = 2.36 ± 0.26 ( min) PS-St8 25.3/ ± 0.02 PS-L ± 0.30 PS-St8 45.5/364 k 1 = 6.11 ± 1.07 (0-40 min) k 2 = 3.24 ± 0.19 ( min) AM Striegel, J. Biochem. Biophys. Methods 56 (2003)

37 Influence of Chain Conformation on Transient Elongational Flow Degradation SG Ostlund, AM Striegel. Polym. Degrad. Stab., 93 (2008) 1510 AM Striegel. Biomacromolecules, 8 (2007) 3944

38 Cellulose & Amylose Structures HO OH O HO OH ( ) O O n HO OH O HO O HO O OH O OH O Cellulose -(14) Amylose -(14) HO OH m

39 Ultrasonication of Cellulose & Amylose Derivatives Hydroxypropyl cellulose in H 2 O, EtOH, & THF (Malhotra 1982). Nitrated cellulose in EtAc (Marx-Figini 1997). Hydroxyethylsulfoethyl cellulose, sulfoethyl cellulose, & carboxymethylsulfoethyl cellulose and carboxymethyl starch in H 2 O (Schittenhelm & Kulicke 2000). Hydroxyethyl cellulose in H 2 O (Xiuyuan et al. 2001). Methylhydroxyethyl cellulose in 0.1 M NaNO 3 (Pfeferkorn et al. 2003). Carboxymethyl cellulose in H 2 O (Grönroos et al. 2004). No studies of non-derivatized cellulose or amylose.

40 Cellulose & Amylose Molar Mass & Polydispersity Sample M n M w M z PDI Cell 1 28,800 82, , Cell 2 25,100 63, , Cell 3 82, , , Cell 4 121, ,000 1,728, Amyl 1 83, , , Amyl 2 60,000 92, , Data obtained using SEC/MALS, in DMAc/0.5% LiCl. M n, M w, M z in g/mol. PDI = M w /M n AM Striegel, Biomacromolecules, 8 (2007) 3944

41 Differential weight fraction Differential weight fraction Cellulose vs. Amylose a 1.0 Cellulose 4 0 min min 120 min min 420 min 780 min Amylose min 20 min 40 min Molar mass (g/mol) 60 min min 180 min 240 min min 360 min min Molar mass (g/mol) 480 min a Amylose courtesy of Dr. Randy Shogren (USDA) AM Striegel, Biomacromolecules, 8 (2007) 3944

42 M lim : Cellulose vs. Amylose M lim (g/mol) Cellulose ~200,000 Amylose ~40,000 AM Striegel, Biomacromolecules, 8 (2007) 3944

43 Degradation of Cellulose vs. Amylose Cellulose & amylose are both linear polymers. Both are composed of D-anhydroglucose repeat units. Both are (14)-linked polysaccharides. Anomeric configuration: OH O HO O HO OH O HO OH O OH O HO O OH O HO HO O OH in amylose In cellulose

44 Degradation of Cellulose vs. Amylose -(14) linked cellulose known to be a stiffer (more rod-like ) polymer than -(1 4) linked amylose. M cellulose lim 5( Mlim amylose ) REM: Path length effect.

45 Persistence Length, L p L p r The chains of stiffer ( rod-like ) polymers persist for longer in the direction of the first statistical repeat unit than do the chains of more flexible ( coil-like ) polymers. Î 1 I.e., a stiff, rod-like polymer will have a longer persistence length L p than will a flexible, coil-like polymer. L p n Iˆ 1 I ˆ l 1 i1 1, as n We can determine the persistence length L p using SEC/MALS.

46 Persistence Length, L p M M L L M R M L p p L G / 2 1/ 2 L p r Î 1 2 1/ 2 R G M 1/M 2 1/ ) (3 2 3 L p L M L M 2 1/ 3 p L L M L p cellulose = 55 3 nm L p amylose = 11 2 nm AM Striegel, Biomacromolecules, 8 (2007) 3944

47 Degradation of Cellulose vs. Amylose M cellulose lim 5( M lim amylose ) L p cellulose 5( L p amylose ) AM Striegel, Biomacromolecules, 8 (2007) 3944

48 Limiting Molar Mass, M lim Definition #3 For a polymer to degrade, a continuous path of a given length and flexibility must exist within the polymer. This path can be characterized by the persistence length, L p, so that: Given two polymers of the same chemistry, L L p p 1 2 M M lim lim 1 2 For a given set of experimental conditions

49 Toward more compact structure Ultrasonic Degradation: Architectural Effects Theoretical hard-sphere limit 1.0 R,w /R G,z Linear Random Coil Polystyrene (in DMAc/LiCl) a PBLG (in DMAc/LiCl) b Sonication time (min) Amylose Cellulose a AM Striegel, Biomacromolecules, 8 (2007) 3944; J. Biochem. Biophys. Methods, 56 (2003) 117 b SG Ostlund, AM Striegel, Polym. Degrad. Stab., 93 (2008) 1510

50 CONCLUSIONS Macromolecular architecture affects mechanism of polymer degradation in transient elongational flows. Have proposed a revised definition of M lim that includes effects of LCB and conformation on degradation. Results for degradation of rod-like polymers and copolymers (random, block, and alternating) agree with proposed mechanism. Ultrasonic laboratory studies: Can control experimental parameters; require very little sample. Multi-detector SEC yields wealth of data related to architectural and conformational effects on polymer degradation and stability.