Title page for ETD etd-02152007-093909

Type of Document Dissertation
Author Li, Bingbing
Author's Email Address bili2@vt.edu
URN etd-02152007-093909
Degree PhD
Department Chemistry
Advisory Committee
Advisor Name Title
Esker, Alan R. Committee Chair
Crawford, Daniel T. Committee Member
Marand, Herv L. Committee Member
Riffle, Judy S. Committee Member
Yee, Gordon T. Committee Member
  • Langmuir monolayers
  • Polymer blends
  • Crystallization
  • Diffusion-limited growth
  • Dendritic growth
  • Poly(caprolactone)
Date of Defense 2007-01-31
Availability unrestricted
Poly (caprolactone) (PCL) has been widely studied as a model system for investigating polymer crystallization. In this thesis, PCL crystallization along with other phase transitions in PCL-based polymer blends are studied as Langmuir films at the air/water (A/W) interface.

In order to understand the phase behavior of PCL-based blends, surface pressure induced crystallization of PCL in single-component Langmuir monolayers was first studied by Brewster angle microscopy (BAM). PCL crystals observed during film compression exhibit butterfly-shapes. During expansion of the crystallized film, polymer chains detach from the crystals and diffuse back into the monolayer as the crystals “melt”. Electron diffraction on Langmuir-Schaefer films suggests that the lamellar crystals are oriented with the chain axes perpendicular to the substrate surface, while atomic force microscopy (AFM) reveals a crystal thickness of ~ 7.6 nm. In addition, the competition between lower segmental mobility and a greater degree of undercooling with increasing molar mass produces a maximum average growth rate at intermediate molar mass.

PCL was blended with poly(t-butyl acrylate) (PtBA) to study the influence of PtBA on the morphologies of PCL crystals grown in monolayers. For PCL-rich blends, BAM studies reveal dendritic morphologies of PCL crystals. The thicknesses of the PCL dendrites are ~ 7-8 nm. BAM studies during isobaric area relaxation experiments at different surface pressure reveal morphological transitions from highly branched dendrites, to six-arm dendrites, four-arm dendrites, seaweedlike crystals, and distorted rectangular crystals. In contrast, PCL crystallization is suppressed in PtBA-rich blend films.

For immiscible blends of PCL and polystyrene (PS) with intermediate molar masses as Langmuir films, the surface concentration of PCL is the only factor influencing surface pressure below the collapse transition. For PS-rich blends, both BAM and AFM studies reveal that PS nanoparticle aggregates formed at very low surface pressure form networks during film compression. For PCL-rich blends, small PS aggregates serve as heterogeneous nucleation centers for the growth of PCL crystals. During film expansion, BAM images show a gradual change in the surface morphology from highly continuous networklike structures (PS-rich blends) to broken ringlike structures (intermediate composition) to small discontinuous aggregates (PCL-rich blends).

  Filename       Size       Approximate Download Time (Hours:Minutes:Seconds) 
 28.8 Modem   56K Modem   ISDN (64 Kb)   ISDN (128 Kb)   Higher-speed Access 
  AbstractandTableofContents.pdf 164.18 Kb 00:00:45 00:00:23 00:00:20 00:00:10 < 00:00:01
  CHAPTER1Overview.pdf 93.73 Kb 00:00:26 00:00:13 00:00:11 00:00:05 < 00:00:01
  CHAPTER2Introduction.pdf 1.51 Mb 00:06:58 00:03:35 00:03:08 00:01:34 00:00:08
  CHAPTER3ExperimentalMethods.pdf 160.78 Kb 00:00:44 00:00:22 00:00:20 00:00:10 < 00:00:01
  CHAPTER4PurePCL.pdf 323.28 Kb 00:01:29 00:00:46 00:00:40 00:00:20 00:00:01
  CHAPTER5PCLPtBAI.pdf 842.58 Kb 00:03:54 00:02:00 00:01:45 00:00:52 00:00:04
  CHAPTER6PCLPtBAII.pdf 1.21 Mb 00:05:36 00:02:53 00:02:31 00:01:15 00:00:06
  CHAPTER7PCLandGlassyPS.pdf 3.92 Mb 00:18:08 00:09:19 00:08:09 00:04:04 00:00:20
  CHAPTER8MWDependence.pdf 871.72 Kb 00:04:02 00:02:04 00:01:48 00:00:54 00:00:04
  CHAPTER9ConclusionandFutureWork.pdf 1.37 Mb 00:06:20 00:03:15 00:02:51 00:01:25 00:00:07
  References.pdf 161.84 Kb 00:00:44 00:00:23 00:00:20 00:00:10 < 00:00:01

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