Ep20 Block copolymers & Liquid crystals NANO 134 UCSD Darren Lipomi

Darren Lipomi
24 May 201747:51
EducationalLearning
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TLDRThis lecture delves into the complexities of crystallization kinetics and the formation of various polymer structures, including spherulites and dendrites. It explores non-periodic growth, highlighting the creation of single crystals and shish kebabs, and discusses the significance of block copolymers in forming nanostructures with potential applications in microprocessor technology. The session also touches on liquid crystalline polymers, their phases, and their role in display technology, concluding with the importance of the glass transition temperature in determining polymer morphology and mechanical properties.

Takeaways
  • πŸ“š The lecture continues the discussion on spherulitic growth and introduces the crystallization kinetics, acknowledging the complexity of the topic and referring students to page 552 of their textbook for further study.
  • πŸ” The crystallization fraction 'V sub C' and amorphous fraction are defined, with equations provided to describe their relationship and the kinetics involved, including the Avrami equation in different scenarios.
  • 🌑️ The distinction between different types of nucleation, such as simultaneous and sporadic, and their impact on the crystallization process is explained, highlighting variations in the Avrami exponent 'N' and the constant 'K'.
  • 🌐 The concept of non-periodic growth is introduced, describing how polymers can form structures other than spherulites under certain conditions, such as cooling rates and shear forces.
  • πŸ’§ The formation of single crystals, dendrites, and shish kebabs from solutions or melts is discussed, emphasizing the unique structures and properties that arise from these processes.
  • 🧬 The role of block copolymers in creating non-periodic morphologies is explained, detailing how different polymers can self-assemble into various nanostructures with potential technological applications.
  • πŸ”¬ The use of block copolymers in microprocessor technology for creating low K dielectrics and in photolithography to enhance pattern resolution is highlighted, showcasing their practical applications in the semiconductor industry.
  • πŸ“ˆ The Flory-Huggins interaction parameter 'chi' and its role in phase diagrams of block copolymers are discussed, illustrating how different morphologies can be achieved by adjusting the composition and temperature.
  • 🌌 Liquid crystalline polymers are introduced, explaining their structure, the presence of mesogenic units, and their unique properties, such as birefringence, which is crucial for liquid crystal display technology.
  • 🌑️ The importance of the glass transition temperature (Tg) in determining polymer morphology and mechanical properties is emphasized, describing how polymers behave differently above and below Tg.
  • πŸ“… The lecture concludes with a preview of upcoming topics, including the glass transition temperature and its implications for polymer properties, setting the stage for the next class.
Q & A
  • What is the crystallization kinetics discussed in the script?

    -The crystallization kinetics discussed in the script refers to the process by which crystalline structures form from amorphous materials. It includes the Avrami equation, which describes the relationship between the crystalline fraction and time, taking into account factors like nucleation and growth rates.

  • What is the Avrami equation and how does it relate to crystallization?

    -The Avrami equation is a mathematical model that describes the kinetics of phase transformations, such as the formation of crystals from a melt. It is given by V_sub_C = 1 - exp(-K * t^N), where V_sub_C is the crystalline fraction, K is a rate constant, t is time, and N is the Avrami exponent, which depends on the nucleation and growth mechanisms.

  • What is the difference between simultaneous and sporadic nucleation in the context of crystallization?

    -Simultaneous nucleation refers to the scenario where all crystallization centers or nuclei form at the same time, leading to uniform growth. Sporadic nucleation, on the other hand, occurs when nuclei form at random times, which can result in uneven growth and different crystalline structures.

  • What are the factors that determine the value of the Avrami exponent (N) in crystallization?

    -The Avrami exponent (N) is determined by the dimensionality of the growth, the nature of nucleation (spontaneous or induced), and the order of the reaction. For example, with simultaneous nucleation, N equals 3, while for sporadic nucleation, N equals 4.

  • What is a 'shish kebab' structure in polymer science?

    -A 'shish kebab' structure in polymer science refers to a type of polymer morphology that forms under certain conditions, such as from solutions subjected to shear. It consists of long fibers (shish) and lamellar structures with long molecular chains in between, resembling a kebab, and is used to create strong fibers like Kevlar.

  • What is a block copolymer and how does it form different morphologies?

    -A block copolymer is a polymer made up of two or more covalently linked polymer chains that are chemically distinct but do not mix due to their incompatible nature. These copolymers can self-assemble into various morphologies, such as cubic, hexagonal, gyroid, and lamellar structures, depending on the ratio and chemical nature of the blocks.

  • How can block copolymers be used to create low dielectric constant materials?

    -Block copolymers can be used to create low dielectric constant materials by exploiting their ability to form regular porous structures. For instance, one block can be selectively etched out to create air-filled pores, which have a dielectric constant similar to that of air, thus lowering the overall dielectric constant of the material.

  • What are liquid crystalline polymers and why are they important?

    -Liquid crystalline polymers are polymers that contain mesogenic units, which impart rigidity and the ability to form liquid crystalline phases between the crystalline and melt phases. They are important for applications such as display technologies, where their anisotropic properties and birefringence allow for the control of light polarization.

  • What is the significance of the glass transition temperature (Tg) in polymers?

    -The glass transition temperature (Tg) is significant because it marks the temperature at which a polymer transitions from a glassy, brittle state to a rubbery, more deformable state. Above Tg, the amorphous regions of the polymer can undergo significant molecular motion, which affects the material's mechanical properties, such as toughness and viscoelasticity.

  • How do sub-Tg relaxations in polymers contribute to their mechanical properties?

    -Sub-Tg relaxations, such as bond rotations, allow the polymer chains some mobility even in the glassy state. This molecular motion contributes to the absorption of energy prior to fracture, making polymers like PMMA tougher and able to absorb more energy than ceramics before breaking.

Outlines
00:00
🌞 Introduction to Crystallization Kinetics

The video begins with a discussion on crystallization kinetics and mentions the limitations due to the course's duration. The instructor references specific pages in the textbook for further reading and highlights the importance of understanding crystallization kinetics in polymers. The process of crystallization and the associated equations, such as the Avrami equation, are introduced. The instructor explains the difference between simultaneous and sporadic nucleation in spherulites.

05:09
πŸ”¬ Non-Periodic Growth and Fiber Structures

This section delves into non-periodic growth of polymers and introduces the concept of single crystals, dendrites, and their formation. It highlights the difference between spherulitic structures and those formed under different conditions, such as supercooled solutions or shear. The importance of shear in forming strong fibers like Kevlar is discussed, along with the unique properties and structures of dendrites.

10:10
πŸ—οΈ Dendrimers and Single Crystals

The instructor explains the difference between dendrimers and dendrites, emphasizing the covalent bonding in dendrimers. Various crystalline structures and their molecular alignments are described, including the rigid chain structures in semiconducting polymers. The section concludes with an introduction to the shish kebab structure, formed from high molecular weight polymers in solution, and its applications in creating strong materials.

15:13
πŸ”„ Block Copolymers and Morphologies

This paragraph focuses on block copolymers, their formation, and the different morphologies they can adopt, such as cubic, hexagonal, and gyroid structures. The instructor discusses the self-assembly process and how varying the monomer content affects the resulting structure. Techniques for creating block copolymers, such as anionic polymerization, are mentioned, along with their potential applications in nanotechnology.

20:14
πŸ“ Phase Diagrams and Morphology

An explanation of phase diagrams for block copolymers is provided, illustrating how different morphologies are achieved at various temperatures and polymer fractions. The instructor introduces the concept of a cloud point diagram and discusses the various phases, such as cubic, hexagonal, gyroid, and lamellar. The role of temperature and polymer interactions in determining these phases is highlighted.

25:17
πŸ”¬ Applications of Block Copolymers

The practical applications of block copolymers in microprocessor technology are explored, particularly their use in creating low dielectric constant materials through the formation of porous structures. Techniques like photolithography and graph epitaxy are introduced as methods to enhance pattern density in microelectronics. The importance of regular porosity for dielectric materials is emphasized.

30:24
πŸ“ Liquid Crystalline Polymers

This section introduces liquid crystalline polymers, their structure, and the concept of mesogenic units. The instructor describes the phases between the crystalline and melt states, such as smectic and nematic phases, and their unique properties. The birefringence of liquid crystalline polymers and their use in display technologies are also discussed.

35:24
🌑️ Temperature Effects on Polymer Phases

A detailed explanation of the temperature effects on liquid crystalline polymers is provided. The instructor outlines the transitions from glassy to crystalline, and eventually to melt, highlighting the unique properties of liquid crystalline phases. The importance of these phases in various applications is discussed, including their use in controlling light polarization.

40:24
πŸ“š Course Review and Exam Preparation

The instructor provides a brief review of the course content in preparation for the upcoming exam. Important topics covered in the course, such as polymer crystallization and mechanical properties, are summarized. The instructor also outlines the schedule for the remaining classes and the special topics to be covered in the final week, emphasizing the importance of attendance.

45:24
πŸ§ͺ Glass Transition Temperature and Polymer Morphology

The final section of the video discusses the glass transition temperature (TG) and its significance in determining polymer morphology. The instructor explains the difference between glassy and rubbery states, and how polymers absorb energy prior to fracture. The importance of sub-TG relaxations and the mechanical properties of polymers above TG are highlighted, concluding the main part of the course content.

Mindmap
Keywords
πŸ’‘Crystallization Kinetics
Crystallization kinetics refers to the study of the speed at which crystals grow or form from a solution or melt. In the video, the concept is introduced with the discussion of the Avrami equation, which is used to describe the transformation of amorphous material into a crystalline structure over time. It is central to understanding the formation of different polymer structures.
πŸ’‘Amorphous Fraction
The amorphous fraction is the portion of a material that does not have a well-ordered, repeating structure. In the script, it is calculated as one minus the volume fraction of crystalline material (V_sub_C). It is important in the context of the video because it helps to characterize the material's properties, such as its rigidity or flexibility.
πŸ’‘Spherulitic Growth
Spherulitic growth is a type of crystallization where polymer chains form spherical aggregates with a radial structure. The video discusses how spherulitic structures do not always form uniformly and can vary based on the crystallization conditions, such as simultaneous or sporadic nucleation.
πŸ’‘Nucleation
Nucleation in the context of the video refers to the initial stage of crystallization where small regions of ordered material form, acting as seeds for further crystal growth. The script differentiates between spontaneous and simultaneous nucleation, which affects the overall crystallization kinetics and the resulting polymer structure.
πŸ’‘Dendrite
A dendrite is a type of crystal growth that results in a tree-like structure due to the diffusion of molecules and their addition to the growing facets of the crystal. The video mentions dendrites as an example of non-spherulitic crystallization that can occur under certain conditions, such as supercooling.
πŸ’‘Shish Kebab Structure
The shish kebab structure is a type of polymer morphology where long fibers (shish) are surrounded by lamellar (sheet-like) structures. The video describes this structure as being formed from solutions under shear, which is important for creating strong fibers like those used in Kevlar and fishing lines.
πŸ’‘Block Copolymer
A block copolymer is a polymer consisting of multiple blocks or chains of different types of monomers. The video explains how these copolymers can self-assemble into various nanoscale structures, such as cubic, hexagonal, and lamellar morphologies, depending on the ratio of the different monomers.
πŸ’‘Gyroid Structure
The gyroid structure is a complex, three-dimensional morphology that can be formed by block copolymers. It is characterized by its interconnected network where one polymer can be traced through the structure without crossing into the other polymer phase. The video mentions the gyroid as one of the possible structures formed by block copolymers.
πŸ’‘Liquid Crystalline Polymers
Liquid crystalline polymers are polymers that exhibit liquid crystalline phases, which have properties between those of conventional liquids and solid crystals. The video discusses these polymers as having mesogenic units that give rise to ordered phases with potential applications in display technologies and other areas.
πŸ’‘Glass Transition Temperature (Tg)
The glass transition temperature is the temperature at which a polymer transitions from a hard, glassy state to a more deformable, rubbery state. The video explains the significance of Tg in determining the mechanical properties of polymers, such as toughness and viscoelasticity, which are influenced by whether the polymer is above or below this temperature.
Highlights

Discussion of crystallization kinetics and the Avrami equation, which is crucial for understanding the crystallization process in polymers.

Introduction of the crystallization fraction parameter V_subC and its relation to the amorphous fraction in polymers.

Explanation of different crystallization scenarios including simultaneous and sporadic nucleation and their respective equations.

The role of radial growth rate R and the number of nuclei n in the crystallization process.

Formation of non-spherulitic structures such as single crystals and dendrites in polymer crystallization.

Distinguishing between dendrimers and dendrites, highlighting their structural differences and formation processes.

The concept of 'shish kebab' structure in polymers, which is significant for creating strong fibers like Kevlar.

Different types of block copolymers and their self-assembly into nanoscopic patterns with high regularity.

The impact of molecular weight on the formation of 'shish kebab' structures and their application in high-strength materials.

The variety of morphologies formed by block copolymers, including cubic, hexagonal, gyroid, and lamellar structures.

Applications of block copolymers in microprocessor technology, particularly as low K dielectrics.

The use of graphoepitaxy with block copolymers to increase pattern density in photolithography.

Introduction to liquid crystalline polymers and their phases, including nematic and smectic.

The birefringence property of liquid crystalline polymers and its application in liquid crystal display technology.

The temperature-dependent phase transitions in liquid crystalline polymers from glass to crystalline to liquid crystal phases.

The significance of the glass transition temperature (TG) in determining polymer mechanical properties.

The practical applications of polymers in drug delivery and other fields, to be discussed in upcoming classes.

The importance of understanding sub-TG relaxations for the mechanical properties of glassy polymers.

Transcripts
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